If they cut and shortened the drive shaft it might not be balanced right, it also has marks on it as must go back in the same position as it came out.
I would have it checked first as it is the simpler of the possible problems.There have been a lot of threads and different opinions on this conversion usally it's putting a 240 tranny in an automatic 617 car which is about the same thing, so you might want to check out search......

William Rogers...........