340 misfire

I have a '73 dart sport 340 4 speed that has a misfire that I cannot get rid of. I have been through 2 sets of plugs, new wires, new cap and rotor, 2 different carbs, different timing, and nothing has helped. It does it between idle and around 2000-2500 rpms. After that, I don't know if it goes away, or I just can't hear it anymore. It has a stock electronic distributor with a msd 6al box and msd coil. It started after I cut off the exhaust to where it dumps at the axles. A week or so earlier I put a new cam and intake on it, but I took the valve covers off and watched the rockers, and everything seems to go up and down like it should. I have checked the plugs many times, and at first, it was the number 2 cylinder, and now it seems to be the number 8. I can tell it comes from the right bank because of the exhaust. I hooked a vacuum gauge to it and it pulls 6 inches of vacuum, and doesn't move much at all. I can tell it is down on power. It has drag radials, 4.10 gear, j heads, rpm air gap intake and the fastest it's gone in the 1/8 is 9.41. I keep thinking it could be a burnt valve, maybe a vacuum leak, or some kind of ground. I really want to get this thing running right so I can take it to the Dragstrip, and drive it to school this year. If anybody has had the same issue or has any idea how I can fix it, I would appreciate it. Thanks, Logan