Sbc vibration at 3000 rpm

[Cole1969]

I have a 1969 el Camino with a sbc 350 and a th350 and just rebuilt the engine in it after it had been sitting for a while. Right when I stopped driving it a couple years back it started to develope a vibration around 3000 rpm and now just recently I pulled the engine and trans and rebuilt the top end of the motor replacing heads, cam lifters, intake, Tourqe converter, but I kept the water pump, alternator harmonic balancer, carb, distributor, and pretty much everything from the converter back, so it only seems to vibrate when in gear it's not speed dependent. I kinda think it would be either something with the drive shaft or maybe the balancer. But I'm also using a short water pump that is who knows how old could that be causing the problem? Any ideas are appreciated thanks