I would recommend a complete tranny.
For right at a grand you can have one shipped to your door ready to bolt in.
Far cheaper than building one from scratch IMO and a whole less headache ordering parts, cleaning & re-assembling.
BTE, TCI, T/S all have entry level powerglides that are rated for around 800 HP just shop around and compare.
I ended up with a abruzzi racing entry level powerglide, they list in the classifieds.
I'm very pleased with it.
Cp