Welcome to CHR!!!! Have you taken some of the dimension off the '48 body??? If you're going to build a frame from scratch you might do better with a frame that would have the correct dimensions for the body you're using....or if you have the frame for the '48 you might be better to start with it. As a first time build, might want to keep things simple so the project doesn't end up overwhelming you and get pushed in a corner and forgotten. I'm not trying to discourage you, just saying what we've seen happen in the past.

Plenty of experience on the forum here to help you out with difficulties you'll encounter during the course of a build.