I think the easiest way is to use a 79 F100 4x4. Replace the uni floor (Cab portion) with the floor from the 79. Fabricate the rear cab and bed mounts to adapt the uni body to the 79 frame. None of this is going to be easy. The cab floors are very similar on all 61-79 Ford trucks. Using the 79 floor gets you front cab mounts that bolt to the 79 frame. Fabbing rear cab mounts out of 1x2 tubing is not complicated. I have not done this swap but I intend to do it when I find a suitable Unibody truck. I have a source for a 61 uni long small that I may cut up just to see if this will be feasible. I saw a short uni on a 79 4x4 frame in Georgia. That truck looked so stock I went and spent abkout a hour looking at it. He used all the 79 running gear. IMHO this id a dooable deal!
No! You are going to set the wheelbase of the newer frame to match the wheelbase of the Uni. So it doesn"t matter what frame you use. When I do frame swaps I take both the old and the new to the welder and he matches up things like cab mount points and wheelbase. Then I do the sheetmetal. I like swapping in complete new floors because it is the easiest way to get cab supports to line up.