Diff ratio help

Hang on fellas;
As to fuel Economy;
1) If that engine has a factory computer controlled EFI/Timing, then run whatever rear end ratio that computer was tuned for.
2) If that engine has a stand-alone EFI/Timing computer, well yur in for a treat tuning that for 3.07s.
3a) If that engine has a carb, AND mechanical distributor, and has long-tube headers; I foresee lotsa problems.
3b) To get as much power as you say your engine has, IDK the cam size/Dynamic compression ratio/ nor the cylinder pressure, cuz I don't run in those circles......
But I can tell you this, from my fairly extensive modest-cam experience;

So from here to the end of my post, this is all from my experience, and I get that nunuvit may apply to your application. It's OK, I'm retired, so how much typing I do, is not up to Rusty.

For a modicum of hiway fuel economy,
1) as to Power stroke; the cam should have more than about 104*. This is already too low for my liking. It doesn't get much better until 110* . And, honestly, 114* is where I'd like to be.
2) As to overlap; by the time it gets to ~60*, the headers, at low-rpm, are pulling fuel charge right across the top of the piston and out the back door it goes. By 76*, you better have the timing figured out, or you'll be leaving a trail of stink in your wake.
3) As to cylinder Pressure, the more the better, until you get into detonation.
4) As to intake and Exhaust durations, you get whatever is left over. If the engine doesn't make enough power on the leftovers, get a bigger engine, or supercharge it.
As an example;
say you had a cam with 108* Powerstroke, 120* compression; that totals 228* . The total available is 720 plus say 76* overlap=796. So then, that leaves 568* for Intake plus exhaust; so say 284/284, or 280/288. On a fast rate of lift FTH cam that might be 240/240@050. That's getting to be pretty big for a Manual trans car.
5) What I figured out for my COMBO (see Avatar) was to run the hiway rpm at just above the rpm that generated the highest no-load vacuum, at the highest possible ignition timing.
This is the lowest rpm that the engine first becomes fuel efficient.
Any lower, and
a) some of the fuel-charge is going right out the exhaust, b) or the Short Power-Stroke is blowing wasted energy right out the tailpipes. Once the engine is built, you cannot do much about the PowerStroke except degree the cam to maximize that.
Any more than this, and
a) the rpm is multiplying all the shortcomings of the combo. and
b) yur just wearing out your engine for no good reason.

6) to that end, I developed an easy test.
I mean it's so simple.
Install a vacuum gauge and SLOWLY, rev the engine up, until the vacuum plateaus, then, increase/decrease the timing until the engine looses rpm, then reduce the rpm say 500 and again slowly bring the Rs up, until the rpm again plateaus, and again adjust the timing for highest rpm.
Record the Timing and Rpm.

My engine has had three different cams, and on each camchange, I changed the compression ratio, to arrive at or near the previous pressure. So, when I did this, the vacuum plateaued;
....~2400 with the 292/292/108 cam,
to ~2000 with the 276/286/110 cam,
to ~1800 with the 270/276/110 cam.

In each case, I initially geared the car to those minimum rpms, using a combination of rear gears and transmission gears.
Some combinations had 8 useable progressive gears, some had 7, and I now have 5, cuz that's all my 367 needs. Eight or even Seven was just too much.

Now, That method above, establishes the ideal rpms.
Sometimes, I imagine, the minimum rpm might be rather high. Say your minimum turns out to be 2600, but you want to cruise at 1800. The logic argument is that as the rpm goes down, fuel economy goes up, and that is true for a small-cammed engine , like a factory 318, that vacuum plateaus just off idle, lol.
But say you have established that yours Plateaus at 2600. That is where it will get best fuel economy, jus saying. If you cruise at a lower rpm but same speed, that engine will lose fuel efficiency, but gain economy from the lower rpm.
I proved this once by gearing my 270-cammed combo down to 65=1600rpm in double-over, which came to either 2040 or 2240 in one of the other two selectable First-overs, (Gear Vender, or A833 over) on an 1800rpm plateau . Thus, cruising at either 1600 or at 2040, economy was about the same cuz neither was at the 1800 ideal. But cruising at 75=1800 was, and again, the increased speed cost me what engine efficiency gained.
So that was Problem #1, hiway rpm

Now, about problem #2; how slow do you want to go?
1) Do you need to drive slowly? If so, what is the lowest rpm that your engine can comfortably idle at? You use those numbers together with the tire height, and your minimum roadspeed to calculate your starter gear.
Your tires are about 28 tall.
You may want to parade at 4 mph.
Say your engine idles at 950rpm
Say your trans has a low gear of 2.87.
Doing the Calculation, spits out a Rear gear of 6.89.
That's a Problem;
this car will NEVER parade with people walking.
Lets say you did install those 3.08s. Your minimum roadspeed would be ~9mph, so I mean, that's even fast when pulling thru a drivethru. Yur gonna be toeing the clutch a lot. And your Hi-Perf clutch is gonna have to deal with that.
But say you get the Idle-rpm down to 780;
and run 4.88s;
Now you can drive at a minimum roadspeed of 4.64mph.
But say you got the Wide-ratio trans with a 3.27 low, and still with the 4.88s, Now you can Idle down the road at 4.1mph, still too fast for parading, but acceptable to idle around at carshows.
Lets work the Formula in reverse; with the 2.87 low, and 4.30s what would the Idle-speed need to be, to drive at 3.5mph?
Answer 518rpm. It'd never gonna happen is it......
And with the 3.27 low?
Answer, 590rpm. Still not realistic for an engine of your caliber. If this matters to you, then you kindof need a 2-speed rear-end like a Quickchange.
Ok so that solves those two problems

Problem #3 is this.
In my experience as a streeter; 65 mph is all I wanted to hit, and the only way to get there quick, was in Second gear. If I got into 3rd, the car lost ET. If I tried to get there in First, the rpm went outta sight. So second it was. So I geared it to to just run out of pulling power at 65. Of course that required a different Pumpkin for each cam.
My favorite cam was the little 270* cam, which made up to 195 psi cylinder pressure, so she was no slouch in those first two gears. The PowerPeak was around 5200, but with the Edelbrock heads, she kept pulling a looooog way beyond that. So I installed 4.30s, which got me 6000= 62mph.
Of all the combos that car went thru, this was my favorite.
If you run the close ratio trans, I think Second is 1.89 to my 1.92, so you'll want a similar rear gear, namely the 4.30s. Which will idle at 5.28 mph@780 rpm, in 2.87 low gear..
So again, you need a two-speed rear gear, to solve all three problems so far. Or, an automatic transmission.

(And that's why I pulled the 292 cam outta my Combo. They told me that cam was too big for street, but nobody could tell me why, definitively. I tried ALL SUMMER to make that cam work, cuz at 7000 that thing was still a beast! I pulled it out...... cuz NO-WAY was I gonna put an automatic on that engine. )

There's no sense in me suggesting what rear gear to run, cuz I don't know what's most important to you; and In your case NO RATIO is gonna cover all driving conditions.
About the closest you might get is;
1) at the bottom; the Idle-Speed will be too high for walking speed, no matter what the rear gear is. With 4.30s the minimum Roadspeed is about 5.27@780rpm.(2.87 low)
2) on the street with 4.30s and 28" tires, in 1.89 Second Gear; 65=6330.
3) to cruise at 65=2200, requires a Final-Drive Ratio of 2.82. With 4.30s, that would require an overdrive of .66

Nice car!