There are several companies that make custom tanks. I found them researching bolt in/weld in efi tank cutouts to retrofit my tank. I'd have to dig around for the info. Also remember the aftermarket fuel cells typically use a Chevy fuel sender, so ohms matter depending on your gauges.
These guys have flanges and tanks pre-built for Factory Five roadsters, would not be much of a stretch. Our tank is really simple. Mine is still out on a shelf.
https://fueltankparts.com/collection...parts-adapters