A stock stang can break the tires loose there must be something wrong
I realize that, but when you add larger heads and a cam with larger duration, you move the power to the right, same power plus more can be made, just at a higher rpm. When a cam is used with a IVC point later after BDC, its takes more compression to make up for the lower RPM loss in power. As well as steeper gears. If the cam was shorter in duration, with the IVC closer to BDC, the cylinder pressure goes up as well as the power in the lower RPM's.
Even with the stock heads, the F303 cam likes to be around 3000rpms. Now add a larger CSA head and the power is shifted even higher. You have to cam accordingly, which does not mean a larger duration cam all the time. Unless you want a high reving power band you have to keep the duration lower when the heads get larger, not the other way around. So to compromise, you add more gear to get in the power band quicker, but that just makes a peaky engine with short power band in most cases.
I'd rather see 4.10's with the F303. But even with 3.73's an assuming a T-5 trans
that's still. 12.50 drive ration in first gear. Should blow the tires off there. 266hr or 258hr cam would do a lot better as far as an OTS cam.
So he hasn't lost power, he just has it in a a different spot.