Befuddled with engine problem

[Jake1074]

I have a '67 C10 with a 350 engine and TH350. Not sure of the age of the engine as I just recently acquired the truck. It was running fairly rough and hard to start but it seemed that a simple tune up would correct it. The engine is supposed to have 4,000 miles on it since a full rebuild, but I have no specs. It was claimed however to have a choppy cam. It's also hard to start when hot, but that could be because of the headers.

So far I have replaced carb with a new Edelbrock 1406, fuel line and filter, HEI cap, rotor, coil and spark plugs, timing set at 12. The distributor is almost touching the intake in order to get that much advance. The spark plug wires have not been replaced.

#6 plug had no gap (as in pic) after replacing it and running the engine the gap has stayed to spec.

The carb does pop on initial acceleration and under heavy load. Vacuum is 15-18 and fluctuates. I do think I hear some valve train noise so I guess the V/covers are next to come off.

What am I missing ? Wiped cam lobe? Valves not properly adjusted? Replace the wires next? Compression test? It does drive ok but doesn't have the power it should.

Thanks in advance for your help....