In GT class of racing, two separate fuel tanks is not that important . You can balance the car other ways with ballast.
It is more about access to fueling in long endurance races in the pit stops.
On the Group 4 race cars I honestly thought that the passenger side "filler cap in the gills" was to access the cooling system.
The Panteras are not GT40's and you really can't make one into one. Don't try to.
I can appreciate the thought of not wanting to cut up the original gills. I hated the gas filler neck under the deck lid and moved it to the left side like the later cars.
I do remember that article you wrote Jack. Now that you remind me. At one point I was thinking what I could mold the gills out of?
When I got all the factory parts to do the swap, they were still available new and I got the trap door hinged gills with the package. As it turns out, I hated the rinky dinkness of it and installed a pop-open filler from a 427 Cobra Comp car instead and polished it.
In order to accomplish this I used a 4 or 5" hole saw and just bored through the original gill. All these years later, that filler still sits there and the trap door gill got sold off years ago. As a matter of fact, I think the gill itsell got sold for $500 and there was a lot of grumbling about the price. It was still cheaper then buying from the vendors though.
The gills are very soft cast aluminum and very easy to bore out with a hole saw.