360 balance problems

Find out if he internally or externally balanced it. If he only had the factory balancer and it was external then you need the correct flex plate. (B&M makes it)
If he internally balanced it and didn't have a balancer, and you used the factory 360 balancer, you need a new balancer. If the whole car shakes, the pump seal blew out already, and it doesnt start being felt until 2500, my money's on the flex plate.