What springs are you using? That is to much lift for the stock GT40P springs to handle. Get the Ford Racing spring upgrade at the least (M-6513-A50).

I used GT40P heads, a Comp Cam XE264HR with .512 lift and 1.6 rockers on a bone stock '88 302 shortblock. I was able to reuse my stock 302 push rods, but the block has not been decked and the heads were new also. I orginally had a Cobra intake, 65mm TB and a fender well cold air kit on it. It was still Speed Density and it ran great. That combo gets good mpg, has great drivability and runs strong for what it is.

I got my heads on line from Tri State Cylinder heads. They dont have new castings for the P heads anymore, but they still sell the same package with used castings.

GT40P heads, w/ 1.94 I, 1.54 E, .550 lift dual spring package, with decent seat presures for this cam range.

No matter what anyone tells you they have done, YOU will need to mock up you combp and measure it.

Dont take the risk and throw away your money!