I bought a 1970 GTO with a rebuilt 400. The 400 is 72 YT block with 4x heads on it. Is there a lot of difference between a 69 YT block and a 72 YT block besides original heads? Are the 4x heads any good if I'm looking to get somewhere near 400hp or should I be looking for a different header setup? I really haven't dug too deep into the engine with the car being in storage so that's all the info I have on it.