I don't run dual fuel tanks (maybe some day...) but some decades ago I redid the whole cooling system near the engine. I ran a 2" pipe up from a big welded aluminum surge tank sitting on the right rocker panel up to the right gill, with a rad cap on the end. The stock overflow tank is now a small plastic windshield washer tank, with a 'super-trick racing catch-can' (a 16oz Budweiser can).
Because I didn't want to cut up my original right side die-cast gill if the idea was a failure, I made a silicone mold of it, reproduced a left gill in epoxy/fiberglas and cut that one up. I did have to fabricate a hinge by copying my left side stocker as a mirror image. All this was successful after some tweeking.
So my radiator cap is now under a stock-looking matching access door in the right gill, along with a custom length dipstick up from the oil pan. Good for my aged back- the decklid does not need to be opened to quickly check oil or coolant levels, or add coolant. Without the 2 stock tanks and the A/C lines, (now front- mounted), there's much more space around #4 spark plug and the decklid shocks, and the car Iost another 14 lbs of rear end wt.
Once again-- all of the mods I've done to Judy's long-suffering Pantera L have been written up in various illustrated POCA Newsletters over the last 30 years, and all are in the searchable POCA Archives for viewing or download. Don't ask exactly which ones- my memory is pretty good but not THAT good!