340 Timing / CAM Issue

Vacuum leak, but..........

I'd start with ALL the basics. Check the valve gear even though compression seems OK, and take a look at the valve springs

Along with timing, check that the advance mechanism is working, and are you? (you should not be) using manifold vacuum at the advance. In fact, you might leave the vacuum disconnected until you get this sorted out

Take a good look at rotor, cap, and check the plug wires. Run a "backyard" cylinder balance test, IE short each plug one at a time up at the dist. and look for each cylinder to dump the same RPM or vacuum

Do the carb screws have good effect? My first inkling is a vacuum leak, and don't forget, that can be into the valley area as well

By the way, when you check cam marks, when they are "by the book" at 6 o'clock and 12 o 'clock, that is no6 ready to fire, NOT no1. This is why you thought the rotor was 180 off. The engine won't run at that, and in fact is why you got the fire through the carb.