There is another option. RB's sells a complete replacement frame with suspension for under $8,000.
I am a proponent of the clip approach because most the people who claim the Mustang II is better base the argument on cost, which favors the clip if you do it yourself, and on problems with poorly executed clips. I've always said poor craftsmanship will hurt either. Your story proves it.
The F body cars are a little wide for this application. G body cars like a '78 to 80-something Monte Carlo are better track-width wise.
As for transferring to a later frame: F body does not have a frame. Its a unit body with a front subframe. Most modern cars that do have a frame use a perimeter type frame which is too wide for your truck under the doors and running boards.
A S-10 frame does fit, and one model is very close wheelbase wise, and easy (relatively) to adjust.
I apologize if I'm criticizing your uncle or your best friend, but I'm worried about the judgement of the person giving you advice based on the frame swap suggestions he made. If that is what he suggested you might want to have the whole truck looked at by someone else. I say this because his suggested frame swaps are not workable, and so I doubt he knows what he is talking about. Sorry...
