thats a good article, we change 7 and 4 due to the load put on the cam and the timing that it is put on it, with the size of the cams we run we use a valve spring that has just over 400lbs of seat pressure closed valve, and almost 1300lbs open valve. this was breaking the cam with the stock bbc firing order so when we swapped that it fixed it, its a pretty common thing now in drag racing engines anyway