When you put a new 350 you have oil pressure but no oil to the rocker arms why?

Cam bearings installed wrong, ( covering the oil gallery holes up)

The push rods are stoped up.

DID YOU PRIME THE ENGINE OIL SYSTEM ???

I think you did not let it run long enough for the lifters to filll up with oil.

There is also 3 oil gallery plugs in the front of the engine behind the timing cover

right behind the cam gear. If they weren't put back in then the lifters won't fill up with oil. It will just push the oil out the front of engine. ( NO OIL PRESSURE ON TOP )