Does it have a stud girdle? If so make sure it has enough clearance for rockers to open and close.
Did you pull rockers and make sure they are not hitting retainers on bottom of rocker body? Pushrods wrong length and it can happen..... pull valve covers and start it, some roller rockers tend to hit valve covers and you might need to clearance them in a few spots.
Are you sure the valve springs aren't hitting coil bind? I reckon you may have checked all the geometries by now. But if not, like Kirk says, pop a cover and roll the motor over slowly and check the valves at full lift. Each one. Then do the other side. Unless you've double checked all that already.
You are using the EO-IC valve lash method, yes?
Mechanical fuel pump?
ACES 3112/Team Chevelle Gold #62 Be big, be a 'builder'!