So it's in the engine. Ugh! With an old 'scope you could do a cylinder balance test, but it's smooth at idle, so I expect that it would show even compression. This also means that the carb idle circuits are good and there probably is not a vacuum leak. Things I've seen that cause this: Try bringing up the idle to where it gets rough (1500-1800 should be close to fast idle cam speed when warm). Try richening the carb a bit on each side (either cover with a rag or squirt some carb cleaner in) to see if you have a transition circuit or main circuit issue. I've also seen weak valve springs cause this, but they were old, not fresh. The oscillations at idle they could handle, but not at 1800 RPM. Disconnect the vacuum advance (if equipped). I've seen a strong advance curve (vacuum and/or centrifugal) cause this. You can also rotate the dist. a bit (at the rough speed) to see if it helps. Hook up a vacuum gauge. See if the vacuum gauge fluctuates (it probably will). This tells you it's in the top end, not compression, provided it's smooth at idle. Hook up a timing light. See if it jumps when rough. This would indicate an ignition problem (spark scatter or trigger problem). This could be as simple as a poor ground, or bad cap, weak coil, bad gap on the dist, bad pickup, or bad EI box. BTW, you should be able to remove the entire front clip as a unit unless there were big changes done to the mounting. Drain the radiator and disconnect the hoses. Remove the hood, bumper, rear fender bolts and the bolts below the radiator yoke. Remove stuff attached to the inner fenders. Tape up the paint around the back of the fenders and disconnect any fender struts. With a friend or two, lift the front of the fenders up, carefully spread the rear of the fenders, lift the clip and walk it off forward. 1942 shown, but very similar. Yes, I cringe even mentioning this, but with a lot of care, it can be done. I've had my '40 clip on and off many times, but it isn't freshly painted.