Did you have it running with that transmission?
As strange as it sounds my 360 had a 904 behind it with no weights on the converter. Don't know if they did something inside the converter or what because it ran fine and came out of a car from a friend's Dad who owned it for years and had that transmission put in it.
The second I put a different transmission behind it you could tell it had issues and I had to get the weight added on to it.