Rear Thanks

It's easily possible to put 295s on 15 x 10s into the factory 68 Barracuda tubs. Most of those I have seen come with a 4.5 inch backspace . All you have to do is move the springs in under the inside tub-wall, and narrow the rear to fit. Starting with a bbp rear makes the most sense.
My 68 even accepted 325s on 15 x 10s
IMO your Dana 60 pick-up rear is a wise choice. And the bonus with that is that it will take a lot of peg-leg abuse,lol.