73 dart wide wheels/ backspacing questions

ok im sure somebody can post a link for me as to what im looking for i just couldn't find it!

my dad has a 73 dart LBP with single piston discs on the front, 7.25 in the rear...

we wants to run the same size tire all the way around but wants as wide as possible... but the rims all need the same bs so he can rotate them...

im wondering if swapping a b body (1965) 8.75 will widen the trackin the rear enough so he can run like a 15x7 4.25bs all the way around, light shimming ok...

also wants a 235/245 x 60 in there? what are your thoughts

thx