On Stud girdles. I usually put them near the top of the locking nuts. That way You won't have a chance that the rocker will hit them.
I agree with the others you are either binding springs or hitting valves on the pistons, I am leaning more towards spring bind though.
Again are your springs correct for that lift and as important, are the springs installed to the correct height? What OD springs are you running with that set-up. They should be at least 1.55 springs and probably be installed at about 1.9 to 2 inches installed height. You may have the wrong spring retainers on their also. What brand cam are you running? See if you can get the specs on the spring installed height and recomended spring retainer.
I am running a very similar setup with a little less lift than you and have never had a problem with rockers or springs or valves.

I run timing on my 377 at 36 degrees btdc total timing, that's about where most folks are running the small block chevy's. At least it's a good starting point.