engine swapping?

[Dirtkid93]

I've got a 64' c20, met a guy that has half of a 61' big window ( he literally cut the truck in half behind the cab ) anyway, he said it has a turbo 400 tranny and a 302 in it? said engine and tranny were mine for $200 or buy everything for $500 ( including air bag suspension ) my truck has a I6 230 with a 4 speed granny tranny, if i were to buy his tranny and engine ( been sitting for X amount of years ) and clean them up and get it running, will i have any problems dropping in the v8? are mount relocations necessary? can i keep my 4 speed manual? ( id rather die than get an automatic ) ill probably have to get a new driveshaft at length i need.
and lastly I'm running 4.65 gears in the back, if i gear down to 4.10 or 3.90 will i get better performance with a v8 or what i have now?