360 balance problems

I bought the b&m sfi flexplate for the ext. balanced 360, but the motor was built about five years befor I finally got it in the car and it was then that he told me to get that flexplate, supposedly he spun it with a different carnk since mine was allready iin the car, and he said it would be good, from what he told me the motor was built and balanced for someone else, but they never paid the bill, so he sold the heads off it then he sold me the short block.
I kinda figured I'd have to pull the motor back apart to have it checked, btw the guy that did it retired about two years ago and since then I've heard of other people having proplems with stuff that he did.
so i think that i have a 4500 dollar boat ancor, I wish i woulda went with the 360 crate engine insted.
thanks for the input