Balance Issue / Vibration

340-4spd said
I was very careful about all that - new B&M flexplate, converter looked good - no weights, never had 'em, and made sure everything was to match for a forged crank engine. Now I'm just not sure if I have a bad torque converter or a cast crank engine. And I'm probably confused about the internal.external balance vs. forged/cast crank issues. I always thought the forged cranks were internal/neutral balanced and the cast cranks are external balance.

Ok, I think I have this correct in my mind. It's a forged crank balancer first of all. If the problem started ONLY after the transmission change, that's where the problem is. More than likely it's because you don't have the transmission full of fluid yet. If the torque converter isn't full, it could be out of balance. I'm also assuming you didn't accidentally install a 360 flex plate, right?