Those heads won't help you make big power, but they can run really well on the street with the right gear and the intake should work really well. In fact i am doing one right now that is very similar, but a long rod motor with extra parts I am collecting. It's really a "bench motor" using a bunch of leftover stuff that is coming together to a pretty cool little engine. However, the parts choice is almost identical, including cam (although its a Comp), intake and heads, the big difference is a set of Diamond pistons and a longer connecting rod, but at this power level I wouldn't call that a big deal
The rub though is you'll really want headers to make that thing run well if you haven't thought of that. We are talking probably a 40-50 hp difference from the 390GT manifolds and big numbers across the whole curve, it is significant
The problem is, the only "affordable" headers for that combo are FPAs and you have to specific the GT exhaust flange. Hooker 6114s fit the car well, but the exhaust port on your head is about .250 lower when compared to the bolts, which makes it very tough for headers to seal. Now, people have run them, but I can tell you, changing header gaskets once the engine is in is not where you want to be on a Cougar/Mustang
- 70 Fastback Mustang, 489 cid FE, Victor, SEFI, TKO-600 5 speed, 4.11 9 inch.
- 71 F100 shortbed 4x4, 445 cid FE, headers, RPM intake, 1000 HP series Holley, 4 speed