rebuilt 301 vibrates at around 3k RPM or 40 mph

Getting down to the nitty gritty, I can feel the engine strobing. It has about 500 miles on it and there's probably nothing I'm going to do about it, but what do you think it is? I figure it might be something out of balance in the torque converter? It had been sitting 9 years so maybe crud settled in one part of it? None of the cooling fins were broken.