Have you bought the trans yet? Buying a cheaper trans that has been "Beat to Death" could cost you WAY more than buying one that wasn't abused and spending a bit more on the original purchase. Remember too that the G-56 aluminum case while pretty strong can be ruined beyond repair from some heavy handed, high horsepower previous owner. I speak to you with 45 years experience and found out the HARD way, early on, that buying something already broken, unless you know EXACTLY the damage and costs, will ALWAYS have some other EXPENSIVE surprises!
From the standpoint of doing a conversion, which I have done on many different cars and trucks, is to hopefully have the donor vehicle to have any and all the little attachment parts, any wiring, bolts ect. I did 2 Ford Econoline Van switch overs the other way from std trans to automatics and luckily I had the donor vans as there were a lot of parts needed I didn't think of.