I've got a 73 250 that appears to be original 85k miles. Absolutely pristine under valvecover. It had set for years. Drove it home missing. Rebuilt carb, swapped distributor,points,condensor,cap,rotor,wires,plugs. Still has a fairly consistent miss at idle, at higher rpm no load. It revs clean with no popping etc. Compression test 125-135psi. I was surprised because I suspect it had a sticky valve or leak. It will idle down very low, so maybe a single cylinder. I don't think it is ignition but can't find anything mechanically suspect.