What are your goals? Streetability? Ultimate HP? Budget friendly?
Jay DeLaigle had his '74 TA running in the 10s, looking completely stock with iron d-port heads and log manifolds. BUT… that car was a max effort car, with a lot of time and $$$ in it and it wasn't exactly street friendly- very high compression, requiring expensive race gas to run.
On the other end of the spectrum is my '69 LeMans- again totally stock looking with mildly ported d-ports, log manifolds, untouched '69 intake, stock Q-jet, etc. Has 9.5:1 compression runs on pump gas, runs right at the thermostat, can drive it anywhere- mild and docile, but runs mid 12s at 4050 lb. race weight.
400 hp is easy, 500 is do-able- above that is possible but you're going to be giving up drivability.
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1966 GTO
1969 Lemans Convertible- F.A.S.T. legal family cruiser. 12.59 on G70-14 Polyglas tires. 1.78 60'
1969 Bonneville Safari- cross country family cruiser. .
1979 Trans Am 400, 4-speed, 4 wheel disc.
View from the drivers seat racing down Atco Raceway-
http://www.youtube.com/watch?v=GhYDMdOEC7A
Ride along in the other lane-
http://www.youtube.com/watch?v=mIzgpLtF_uw