Thinking about converting from 4speed to 904 or slant six 727.

I luv the story.
If you're getting to the age where leg power is getting to be a thing; I just want to offer this. That od trans works best with whatever gear gets you to cruising at ~2400 rpm. That means your rear gear will be somewhere around 3.73 depending on your tire size. This will put second gear into an rpm that around town, you almost never have to shift until the Rs fall below ~20 mph.
And your clutch pressure plate preload is adjustable;
And your pedal-ratio can also be modified.
The point is that you are only as married to getting rid of the manual trans, as you allow yourself to be, which may or may not have anything to do with your current or future physical limitations.
Jus saying, lol.
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Having said that;
Personally, I like the combination of a 2800 stall and hiway-type gears.
About 24 years ago, I had a winter motor for my DD Barracuda. Every fall, I would swap from an HO 367 to a POS low-compression 318. This swap was always accompanied by a rear gear swap, and usually from a manual to an automatic, but not every time.
One winter, I ran that A998 with the 2.74 low, the 2800, and 2.76 gears.
For me, that was a fun winter.
That 318 wore a Thermoquad and I installed the TTIs back on the 318 that had come off the 367.
With that combo, I could floor it off the line, get it up into Second asap, (manual shift VB) and wait, no tirespin. The engine would spool up to stall, and just hang there, slowly climbing to 3200@ 65mph, still in Second, the TQ roaring the whole 17 or whatever, seconds. Then into Drive for 65=2230@ a calculated zero-slip, which was actually ~2300/2400 on the tach, so about 3% slip. I loved that roaring poser, pos, lazy-azz lo-compression 318 winter motor..
That car, with the lowered front end, in winter, spent a lot of time being stuck, which I knew would happen cuz of the way I used to drive lol. But with the automatic, the 2800, a posi in the back, and studded tires all round; I was never stuck for long.

That same 2800 used to be in my slanty car, the one that I had when I got married. In fact that 2800 has been in just about every Mopar/automatic car that I have ever owned.
>> IDK if you've ever had one, but if you haven't, lemmee take a second to tell you how mine works;
Off the line, when I floor it, the engine begins to rev up, on the way to the stall. As it goes, it sorta feels like the car is stuck, and somebody has one of those flat snatch ropes laying on the ground between me and his high-powered diesel. When He accelerates away from me, the rope stretches to it's limit, then, the momentum of his truck, snatches my car out of whatever it was stuck in and, Shazzam! the car is catapulted towards the truck ahead of me and we both better be alert. NEVER buy a short snatch-rope, lol.
That's what starting off feels like.
>Now, say I am in Second gear, with a manual-shift VB, just tooling along, at say 1200rpm and I get the urge to accelerate; but I leave it in drive. BAM! the engine revs up, and the tug is imminent. This never gets old........ for me
>Now say I am stuck behind somebody that I want to pass, but I'm not in a hurry about it. So then, I leave it in gear at 55=1900 rpm. The convertor spools up, hits hydraulic lock-up at or near 2800, and watch out, here comes the Tug !
I loved that TC so much, that I took it out of every Mopar I ever installed it into, before parting the rest of it out.
I'm gonna guess I bought it in 77/78, and I still have it ! .......
just not in a vehicle that I still want to drive ........... but I could. I mean if a sweet power-bulge hood, F-body showed up in my driveway.........