Back in the 70's I worked with a friend on a project, we started to build fiberglass 32 Ford bodies, and something we did to eliminate any outside distortion on the body when we glassed support inside the body,...

We rolled up little balls of modeling clay to about 3/16" and put them between the inside of the body and the wood before we glassed it in, as the fiberglass drew in as it cured, the little clay balls would flatten out, the heat generated from curing made the clay very soft, and we never did see any "tattle-tale" marks on the outside of the bodies.

And BTW you can do amazing things with glass bodies that seem rough, you spend some time on that body and it will look fine.