are you swapping v8 to v8, inline six to v8?
sometimes when doing things with the older trucks you may not find parts i.e. bellhousings, so what most people will do is put a th350 or th400 turbo with a v8
i have a 69 chevy truck that had a inline 6, i put a 350 with a th400 in it with no problems with mounts, and i went with trans shifter on the floor, cause the truck had a three on the tree shifter ( on sterring colum for those who don't know the that terminology)