In case anyone wants to replicate how I accomplished this with my limited skills, here's how I did it.
I traced the oval shaped base gasket from my Lemans cap assembly added 1/4" and made a plywood top and bottom. Rolled an oval form out of thin aluminum from Home Depot and wrapped it in air conditioning foil tape, (to make it release easier when cured) Nailed the plywood top and bottom to hold it in place on the aluminum, wrapped it in mat cloth and added cloth/resin until it was 1/4" thick.
After curing, I pulled nails and removed the top and bottom plywood and popped the fiberglass from the aluminum mold. I then fiberglassed the oval well to a flat piece of fiberglass that I had, and cut the center hole for the filler neck. After that, I cut an oval hole in the body and fiberglassed the well to the body, then trimmed off the excess level with the body with an air body saw. I then added filler and sanded smooth the inner sides of the well. Due to the fact that the cap I have has an oval ring on the bottom, I wasn't able to clock the latch like the one is on the alloy car, which has a round bottom. My latch lines up with the center of the quarter panel.
Here are photos of the well fiberglassed into the body, and the cap sitting in the well.
The black in the well photo shows the well that has been fiberglassed over the filler with resin that has been tinted with black dye.
The black on the body edges will be sanded off.
After polishing the Lemans cap and priming the well it will be done.
Thanks again to Jeff who stepped up, measured and posted the photos with the measurements so I could mount it correctly.
Jim