Aftermarket gas tank?

You mean a square gas tank. Not with the area cut into for the spare tire?

Take measurements and compare with other tanks. Food for thought... E body Challengers also fuel up on the drivers side. Holds more fuel than a E-Cuda. Why I don't know but that's what the books say.

I'd like to know if there is a fuel cell that has the ability to install the fuel neck where I want it.
I suppose it could be square, but was thinking keep stock look underneath but add a few gallons.