Car vibration at freeway speeds; why?

Oh another thing too! I bought a 73 D100 that was supposed to be a 318 4-speed. well come to find out someone pieced the poor thing together and put a 318 flywheel, harmonic balancer, intake, and exhaust manifolds on a 360 short block. it shook really bad above 55-60 ish but ran pretty damn smooth at lower speeds. I ended up finding it by the casting numbers on the block, the front pad that should have been stamped was unreadable. So sometimes its the things that most would assume should be right.