318 cam and heads help

Trust but verify. "No machine work needed" unless there IS machine work needed. I would double check valve retainer to guide clearance at full lift. If the guides have been replaced they could be taller than the originals. Also,check the guide to stem clearance, if the valves are too tight in the guides they will stick when the get hot, such as at full throttle.

Since you are breaking rockers and valve tips, I suspect the problem is guide interference. If the valves were hitting the pistons the heads would be getting bent.

If the valve springs are factory 360 springs, you ARE probably experiencing valve float at fairly low RPM with that much lift. Valve float, and minimal retainer-to-guide clearance could cause the problems you are having.