If you rev it to 3000 in neutral sitting still and it does it, you should start at the torque converter. If it has a neutral balance torque converter on a cast crank which requires external balance. If the converter don’t have a big weight on it where flexplate bolts on then it is neutral balance. If that’s the case you can get a different flexplate from b&m that will correct that problem,, or change converter… Does the engine have a factory balancer on the front or an aftermarket? Some of the aftermarket’s will have a small bolt on weight for a cast crank engine…
