2dr post 68 Dart explain..

So I recently bought a 2dr post. Did a little research. I’m new to Mopar’s.. didn’t even know there was such a thing. Also didn’t even know they have a shorter wheel base!?
1) how is the car shorter? Is it from the front door back?
2) is the roof from a valiant?
3) if I had to put 1/4’s on it would I have to get hard top 1/4’s and modify accordingly?
4) are these cars hard to find/ rare?

Car came with build sheet. Looks like the post cars are bare bones cars. Came from Hamtramck slant 6 904 UU1 color, power steering.