AD Chevy trucks are always hot ticket items. Almost like a Model A, you can get nearly everything for it. Putting in a V-8 takes some dinkin around with the steering, but it's done to so many it's almost a no brainer by now. The Ford being a long box, is it also a 3/4 ton?
Wow, imagine if you can two trucks in so-so condition. both needing lots of work and parts. you want opinion as which to buy. I would say neither, since it's sounds like neither of them is something you really want. just available at a low price. think about the hours and the $$ you will have to commit to for either. either of them your ideal dream ride? if you have the $$ to fix one up maybe find something you truly want in better condition.
F-100's 53/56 were the cool cruiser pickups that guys fixed up in the late 60's and through the 60's while for the most part the AD GM trucks were thought of as work trucks. Outside of the Rod & Custom dream truck you seldom saw a fixed up AD pickup until the 70's when I first bought mine and all the sudden it seemed that everyone was fixing one up. It still comes down to what you like and not what we like. Then it comes to which one is within your skill level work wise or within your wallet, cash wise if you pay someone else to do it. It doesn't really matter what engine is in either if you plan to build a fresh motor for it. Just don't pay extra for an engine you don't plan to use unless you have a ready market for it. I looked at what should have been a 400 max parts pickup the other day that the guy wants 2500 for because he has a "good" 235 for it and all I wanted was the running boards figuring that the rest of it was parts or trading material and the fenders were shot and the bottoms of the doors were rotted off. Aftermarket and used pieces run about the same as there are a lot of sources for parts for both. Shortening the F-100 frame should only take a few hours at most and isn't a big deal. The Effie also handles engine swaps a lot better and every performance V8 under the sun has been stuffed under those hoods at one time or another. Being as I have a batch of AD Chevy trucks, cabs and pieces out in the back I'm a bit more inclined to think Chevy rather than Ford though.
I'm neutral on the Ford vs Chevy thing. The way you described them, the Chevy sounds like it's in better shape. If that's the case, get the Chevy. Just make sure with any purchase that you get a clean, clear ***le and it matches the numbers on the engine/truck.