Sounds like you have a good start. The GT40P's perform well with good headers. The carb is a bit big for a street car. The calculation for a street car (idle-6000 rpm) is about 520 cfm so you should stick to a carb somewhere near this for best all around performance, 570 cfm, 600 cfm, okay. The 750 cfm would be great at wide open throttle using a single plane manifold and a good set of long tube headers, else it would be overkill.
The worst part of this mixture is GT40P heads are "Explorer" configuration and finding headers that fit and are cheap is a chore. A couple years ago, there were none. Today there are several manufacturers who make early Mustang headers that fit the P heads.
B303 is a fairly high hp grind used in the late model roller block engines. I don't believe this is suitable for a flat-tappet installation and even were it, the grind is a bit towards the high end. Again, fine with the single plane manifold and large carb but not so great for a street car (okay beat me up)! I would go with a mild street Comp Cam kit.
|