Clearancing Pushrod Holes

Depending on the solid roller spring pressures you will be using, you may not need anything bigger than a good 5/16 wall push rod. I would consult Smith Brothers with your specs to see if it is needed/required. I say that because you may not need to clearance the heads with a 5/16 push rod (I didn't on my 408).

With that said, I would wait until you have answers to all that so you have a definitive game plan, rather than cutting when you don't need to. Just my 2 cents.