1970 Ford maverick 6 cylinder rear axle is an 8" but has 4 lugs. 8 inch is a front loader with pressed on wheel bearings.
If you are going to narrow it then I would start with an 8" or 9" ford.
7.5", 8.5" GM and 8.8" Ford use C-clip axles which are not so good for racing
8.8 rears in the early 80's Mustang GT's were 4 lugs but are too wide.
I priced posi units for an 8" and 9" Fords they cost about the same but the 9" is stronger and a lot easier to find parts for.
For you want, 4 lug x 4", 4.56 posi and 50" or so wide,
you are better off having one built using new 4 lug axles with pressed on wheel bearings,
brakes of your choice and the housing narrowed to your specs so all your wheels fit.