I've got a 63 Fairlane with a 5.0, out of an 89 Lincoln Town Car, that has been carbureted and has an ignition system out of an older model as well. Transmission is a C4.

I started to notice stumbling while in gear and figured it just needed a tune up. The tune up turned into replacing plugs, wires, distributor (with new cap and rotor), and coil. After all this, it still has the same stumbling issue when in gear.
Another thing to note, it runs fine when I start it up cold, let it warm up for a few minutes, and then drive for a mile or two. Only then does the stumble begin.
If I put it in park or neutral, it clears up.
I did notice that even when not in gear, if I rev the engine up to a higher rpm, the engine starts to stumble a little bit. Almost like it's missing all of a sudden at the higher rpm. Drop it back down and it seems fine.
It seems to have gotten a little worse now in that it will die out if I'm just idling in gear. When it first started it would just idle rough, but stay on.

Any ideas out there as to what this could be?

Thanks everyone.

 

This may or may not be the problem.
It does sound like the engine might be starving for fuel at which point you would need to confirm the carburetor is working correctly.
What did the old engine do, and why did you replace it?

 

Another thing to check is the coil. A weak coil will start the engine because the timing is retarded, but has trouble firing the plugs under compression when the timing goes advanced.

 

 

 

I just saw this thread and was thinking it had to do with vacuum. Since it only acted up in gear, I was going to suggest that the vacuum line that goes back to the transmission could've come loose. I've had that happen and it shifts funny as well as kinda staggers at idle. Glad you found it though!