Can I swap my 1966 225 with a 1991 transmission and transmission?

With work you can do the swap but why not keep the truck original? If I were to swap a V8 into this truck I would use a correct engine with a carburetor from the same time period as the truck. A 383 or 361 B series engine would be correct.

They make carburetor manifolds for the late model motors. My biggest concern is mounted and wiring. I would like to know what if any mount mods would have to be made. As as going period correct : I don't have the money to do so. I have been offered a rebuilt 91 roller cam motor for a steal. So this project is very dictated by funds.

The transmission in your truck is the 727 torqueflite. THe 225 slant six 727 had a special bell housing bolt pattern. You would need to change to a V8 727 trans