If you are not going original/stock on your truck, you may get more help posting in the HiPo Shop. I'll try to answer with what little I know.

You need to find a rod shop. Typical transmission rebuilders don't like getting involved with owner conversions or upgrades (owners damaging transmission by improper installation/setup).
A rod shop will have it rebuilt (or trade your core for one already rebuilt) for you then they will install and do setup especially getting the geometry of the carburetor throttle arm right and adjusting the TV cable (throttle valve).

Now I get to preach. I prefer original trucks but if I absolutely had to do a Chevy V8 I would put a TH350 behind it. It's old school but it's reliable and less hassle.
Yes it doesn't have overdrive but driving an AD truck with stock suspension at speeds above 65mph is a "White Knuckle" affair. You can't relax.
Yes, overdrive helps the gas mileage but the AD truck is not streamlined. Like a moving billboard in the wind. Preaching done.

"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."