Performer intake will fit GT40P heads, but remember that the P heads have a revised spark plug location and angle, so exhaust manifold/header fitment can be an issue.
No on the 8" trac loc unit. 85-up Fox body Mustangs used an 8.8" integral rear, not the drop-out-carrier 8" or 9". You need a posi unit for an 8". Some 60s and 70s cars had them, but they're rare. Try Mustang II Cobra IIs, Maverick Grabbers or Comet GTs, or even V-6 Pintos.