1961 Chevy Truck periodic vibration, harmonic balancer

Shakes or Wobbles problem
1961 Chevy Truck V8 Two Wheel Drive Automatic unknow miles

I have a 1958 Chevy Apache Pickup with a used 350 Small Block (unknown year/history, maybe ~'84) and turbo350 automatic transmission. I've been troubleshooting a periodic vibration that seems to be at it's worst at about 30-35 mph. Brrrroooommmm, brrroooom, vibration occurring about every 1-2 seconds at that speed. Slower not a problem, faster it seems to goes away. Straightened driveshaft, new torque convertor, cleaned rear end, new motor mounts, all seemed to help a little, maybe. Then realized I could barely feel it when idling during warmup speed, not in the drive train then. This truck basically sat for over 10 years and I've had almost all the gaskets replaced due to being dried out. Sound like a harmonic balancer?