If you know how to make a fiberglass body then you wouldn't need the dimensions so I'm assuming you don't know how it's done.
Most fiberglass bodies are made by making a mold from an existing body. You put a lot of wax on the body and cover it with fiberglass. Glassing in a wooden or steel frame will reinforce and support the mold when you pull it off the car. Then you wax the inside of the mold and lay fiberglass inside it. It works best if the mold is made in pieces that bolt together.
The other method is the Ed (big daddy) Roth method. He would make the body out of wood and wire mesh that he would cover with plaster. after shaping and sanding the plaster body to perfection he would cover it with fiberglass and then break up the wood and plaster.This gave him the female mold. The one piece mold had to be cut up to remove it from the body so you can only make one body from it.
It's a lot of work but the good news is you can buy a fiberglass body for much less than it would cost you to make one of equal value.