To me it's one of two things depending on the donor car/truck:
1. If you find a SBC donor b/a/g/f-body, then buy L31 long block ($2k), an intake, and a Holly TBI. Return the donor long block as a core, use accessories (alt, flex plate, etc). Use the TH250 until it breaks. When it does, buy a 2004R or 700R4 from Jasper, and change rear gear/carrier to 3.08 - 3.42 Eaton Posi from quickperformance. See if you can bolt up the V6 2-into-1 y-pipe to get it down the road.
2. If you find '98-14 4.8L, 5.3L, 6.0L the main choice is engine mounts. You can get a legit ECM from PSI Conversion for $200 with emissions/VAT delete; mount on top of the inner fender. Replace all the sensors, ebay/Amazon muscle car oil pan, see if exhaust manifolds from a LS1 GTO will clear with your mounts. Problem clearance areas are going to be steering pump vs. steering box and head vs. HVAC. I'd keep it throttle-by cable with OEM LS1 throttle body. If it's got oil pressure and the trans doesn't slip, send it. The details will nickel-and-dime you with this option and you will be lucky to spend < $5K.