As for the rearend lookaround for a Mustang II as they had 8 in fords and you cna make them super tough for cheap.
Figure a set of the
Moser Street replacement axles, it will have bolt in axles, and for a poor mans posi order up an extra pair of side gear shims.
This litterally locks the spiders together but they will still slip some, but off the line the $1.98 posi is an old hot rodder trick.
If you have over drive figure you can use high 3s or low 4s for gears and the rear needs no actual bracing.
A friend had a 71 Vega with a 400 hp 355 and it lived through 2 yrs of racing (on stock axles) and several years of street driving by the guy who bought it.
LAst I heard it was still running the mini spool and 4.66 gears, on the street.
Also it depends on the width of the rear quarters.
As for rear suspension i would look at the triangulated 4 link from
Air Ride.
Honestly some suspension that has available parts like a narrowed Nova/Camaro subframe or looking for one of the Mustang II frontends as you can get coilovers or Shockwaves for them and some sweet parts too.
As its so small it wont take much to make it stick like glue.
anotherthought would be look for a 8.8 IRS from a 89-up T-bird/Cougar.
Pull it in to the extreme and tada it has an awesom upper adjustment in the upper arms for camber too.
I installed one in a Ranger for a guy and it handles like,,,well like a Vette on steroids. Seriously the IRS was super easy to mod and it is seriously strong in light vehicles.
Oh and the rear cover tells you the ring gear size.
Good Luck