rocket - the 400 was available in 70's trucks. I had a 76 with 400ci and th400, that thing was solid. The swap would be really easy, it would also be easy to swap in the 700r4. The swap will not be overly difficult, might have to find or fab a new crossmember? You will also have to get your rear driveline re-sixed. Is this a 4x4 or 2x4? If it is a 4x4 then there will be more to move around. Good Luck.
Kyle
