You could just basically "bolt up" the GT40's to your engine, but may need different length pushrods, and possibly different rocker arms depending on what you have now and how it all sits when it's assembled. My view is if you go to the trouble of taking off the heads to put different ones on, you might as well put a better cam in that matches the heads, but it's not necessary.

 

Thanks for clearing that 67mustang302 I could not see why you would need the cam, but my knowledge is far from extensive when it comes to engine builds