Choosing the best 5.7 engine and Transmission

The more engine you have, and
the broader it's powerband;
the less torque multiplication and the fewer the gears it needs to have.
I have had as many as 8 behind my HO367LA, and as few as 4. It really only needs 3, but they don't make 2+1s. I need two gears to hustle to 60/65mph; and then an overdrive, to cruise with.
So instead, I have a Commando 4-speed mt, with a GVod. Fourth gear is mostly along for a free ride. Even third I could do without . But I like 65=2240rpm.
In a lightweight car, I would do a 3 or 4 auto with a loc-up, and a higher stall; so that the TC acts like a two-speed. Add the loc-up, and the 3-od trans will act like a 5-speed.
Say you had an A999 with ratios of 2.74-1.54-1.00.
And a hi-stall TC with a hydraulic multiplier of 1.6/1
and a rear gear of 2.94s.
Yur looking at a starter gear of
14.50, then~ 10.47-5.88-3.23-2.94 in loc-up.....65=2370rpm with 27s
splits of; .72-.56-.55-.91
Most of the time, you will be driving it 1-2-3L, so your splits will actually look like;
.72-.56-.50 into 3L. As you can see, this combo wants a wide powerband, so you'll need an appropriate cam.
Or
Say you had an A998 with 904 ratios of 2.45-1.45-1.00
And same Hi-stall,2.94 gears.
You might be looking at
~11.52-8.06-5.11-3.23-2.94 in loc-up ....65 still=2370rpm...
splits of .65 -.63-.63-.91.... Needs a different cam than above; and ba-BAM!
And BTW, this trans fits in the A-bodies, like it was designed to go in there,lol. And is cheap to buy,build and rebuild; millions of them are out there.

jus saying.