Slant 6 MPG Upgrades?

For fuel-economy, the best things that you can do are;
>If the engine is coming apart; Plenty of Cylinder pressure, like 155psi, or more.
>Additionally;
>reduced cruise-rpm, together with adequate Cruise timing.
>Then the tune, including; coolant temp, inlet air temp. and
>Finally, is driving style.
>a manual trans is worth about 10% or an easy 2mpg.
> an overdrive is best served with adequate ignition timing. Without timing, it will likely be a disappointment.
> I've had good results with a SuperSix 2-bbl intake. The small primaries do a good job.
> high-altitude will kill your potential.
--------------------------------
IMO,
a GVod may be a disappointment. It is all too easy to get your rpm so low that you cannot, with the factory type distributor, give the engine the timing it wants.... so you may lose more than you gain. Furthermore the buy-in is so large, that it will take many years to break-even.
Last time I checked, I heard that a GVod was over $3000, plus install; and I don't think it will fit in your early tunnel, so extra cost there, plus I think you will have to convert your B&T mainshaft to slip-yoke style, plus a driveshaft = lots of money, for about 28% rpm reduction, and then add in the matching rear gears.
Say you spend $4500. That will buy you an awful lot of gas.......
Say you drive that baby 10,000 miles a year, and the new combo is worth 3 mpg in the long run, going from 20 to 23mpg. That would represent a savings of 65 gallons a year; say $390. Then 4500/390=11.5 years to break-even. See what I mean?

IMO, a good cruise rpm to target is no less than 2200. Any less and you will need a stand-alone programmable timing computer...... like a Lean-burn, lol.
With 2.45s, you would get about 2250@3% slip.
To get that with a GVod and 24.5" tires would take 2.45/.78= 3.14s, rounds to 3.23s or 2.94s.
The problem with 2.45s is that take off will be a tad sluggish. You can mitigate that some by slamming the A999 gears into your A904 pb case. This will get your starter gear back up about 12%, making those 2.45s feel like 2.74s.
Of course the GVod would solve that, with the aforementioned 3.14 gears.
The thing is:
Your combo might have a potential to get 24 mpg when yur done, in point-to-point traveling; but every time you have to accelerate up to speed, there goes a half-pint, because of those 2.45 gears. So that's like 2 or more gallons per tankful, bringing your average down to something like 22mpg. So then you might as well run 2.76s...... whereas with 3.23s you would burn less getting up to speed but more in point to point.
So really, what you need is a higher stall ..... but that pb 904 is gonna be the handicap.
If it was me, and because I have the skills, and parts laying around, I would stuff the A999 guts into her, with a custom LU hi-stall, with 27" tires and 2.76s, for 65=2230. But I don't like that old-style VB so I would get me a post 71VB and convert it to a pb, if I just really had to keep it..... but more likely I would just throw a console shifter on her; which would allow me to run the A999 as is.
Of course the GVod would skip all this too, but with a crazy price tag.
Here's something else;
slowing down 10% is said to gain you 5% in fuel economy, so cruising at 60 should be worth 1 mpg, over cruising at 65. On a 1hour trip, that costs you ~5 minutes, and you saved about .6 qt of gas......... On every tankful, you'll save about a .7 gallon , over 10,000 miles you'll save 24 gallons say $144 bucks a year@$6 per gallon ..... just by slowing down by 5mph.
And finally; jumping your cylinder pressure up to 150/155 from the typical 130/135, is gonna restore your engine's bottom end .....to the point that you can run 2.76s no problem, with your pb904; and so 65=2535@3% slip with 24.5s .....That, IMO, is a daymn fine low-buck compromise.
To compliment that, install the SS intake and 2bbl, and install a free-flow exhaust system.
As to EFI, IDK. IMO again, the change is not worth the buy-in.
BTW
>If you have to cut the tunnel for the GVod anyway, then, IMO, you might as well install a Chevy 4-speed auto, assuming you can get an adapter. They are fairly compact. I know/I know, you want to keep the pb,
>a well engineered 273, in terms of fuel-economy, can easily match your 225, while pulling 2.45 rear gears, for 65=2250@3% slip, which is just about perfect.