Tim, I'm not sure about the clearance but you should definately change the springs when you do it, Jegs has a set of crane springs for about $60, i got my comp's from summit for about $70
when you helped me put my rockers on i didnt notice much gain at all, and after i swapped in the T-5 i noticed i had valve float at about 5400 and higher, when i put the new springs in i could definately notice a good gain in power, not to mention i could rev all the way to the limiter
i dont know how much the springs helped, but with the combination of them, an elec. fan, and VERY cool weather i picked up over 5mph in the quarter (101.4 to 106.8best)
are you gonna be doing it soon? i've got a break from school starting thursday and going all the way through next week so i may stop by some time and see how its goin
good luck , talk to you later, Drew