I recently swapped in a SM465/NP205 combo to replace my TH350/NP205 in my 78’ K-10 shortbed. It was actually a very easy swap considering I had a complete donor truck to work with. My problem is that it has a bad vibration at 3000 RPM since the swap. It feels like the type of vibration that you would get with a worn fan clutch or water pump or when something like the exhaust is grounding out on the frame. Just for the entertainment value, I drove the truck with the accessory belt removed to isolate the accessories. No change. I also had the rear drive shaft lengthened to compensate for the shorter transmission and verified my “Z” driveline working angles are exactly at 3 degrees. I verified with engine casting numbers that both my trucks as well as the donor truck both have 350 engines so I can rule out the possibility of using the wrong flywheel. When I made this swap I installed a new pilot bearing, Centerforce III clutch, pressure plate, and release bearing as well as Energy Suspension polyurethane transmission mounts. The only thing I think could be causing the vib is the polyurethane transmission mounts. When I was in college I was a fleet technician for Fed-Ex and I had a Chevrolet G-Van that had a terrible RPM vibration after a transmission replacement. After spending many hours I swapped the new transmission mount that I had changed when I installed the transmission for the old mount. The vibration was gone and the verdict was the extremely hard aftermarket transmission mount. Possibly, the Energy Suspension transmission mounts are too stiff emitting the driveline harmonics right into the frame??? Anybody ever had problems with polyurethane mounts??? Push buttons are for radios NOT transfer-cases!!!