Check the pressure with a Intercomp valve spring seat pressure tester. This you can do with everything in place. To many people change camshafts and do not check spring rate. I drag race and I check my springs after every race. Now I think you should be safe in the .512 or you might get .550 but don't push it.
Stock gt40 valve train isnt even enough to handle a stock mustang cam. If you are not using an aftermarket cam or INSIST on not upgrading the springs even with a new cam, use the stock e7 valve train. But for 150 bucks for the trickflow kit that you can install yourself its worth it.
ok, not sure i understand. the stock e7 heads/cam had less lift than the cobra cam w/1.7 rockers, right? but you are saying the e7 springs are better? if the gt40 springs wont work i'll get some trick flows but i was just wondering how much lift they will accomodate. i run an x303 with my aluminum gt40 heads with the springs that came with them with no problems. i guess they are not the same springs?
I'm getting my 40p heads installed next week. Using my stock e7 valvetrain and stock HO cam. Will add goodies later down the road, but just want the heads in now.