Engine/drivetrain vibration?

I hope somebody here can shed some light on a possible problem I'm having with the rebuilt 340 engine and/or 727 transmission in my '73 Dart Sport. The engine was rebuilt about 150 miles ago, as was the transmission. I took the Dart into my local shop last week to address a small transmission fluid leak, and after driving the car, the shop owner asked me if I'd noticed a pretty significant "vibration" in the engine or some other part of the drivetrain. The only thing I'd really noticed since the rebuild was a different "resonance" in the sound of the car at highway speeds. I attributed this sensation to the new exhaust system, which was increased to 2.5 inch pipes and new DynoMax mufflers.

Anyway, the shop thinks I should look into the vibration while the rebuild is still pretty new. They determined that the torque converter (replaced during the tranny rebuild) might be the wrong one for a '73 340 (they claim the one in the car is intended for a 360 instead), so they removed two balancing weights on the converter to see if that could be the problem. The vibration is apparently still there. They also checked the harmonic balancer, but it is the original factory part that was reinstalled after the rebuild.

I guess the next step is to have the owner of the machine shop that performed the engine rebuild come over and drive the car to see what he thinks.

Has anyone here had a similar experience? Does a '73 340 with 727 require a different torque converter than a later model 360? If the torque converter and harmonic balancer are not the problem, could the vibration (which you can sort of feel through the seat of your pants) be due to some kind of internal engine problem associated with the rebuild? We also checked the new motor mounts and transmission mount, and they are okay.

This issue is driving me crazy -- I just want to drive the car!

Thanks in advance for any help.