I had to use 2.750 long studs on my Trick Flow heads since I was using the HD style shaft hold downs. The 2.500 long studs might work with a stock stamped hold down?
I used the hold down clamps that came with the Harland Sharp assembly, 2-1/2" worked perfectly. With the valve lash already adjusted and a lot of lift I can see where another 1/4" would be helpful.