I do have a rust free '77 skid plate under my '77 main fuel tank, and I know of no after market replacement tank that will fit in place of the OE tank using the OE skid plate. For anyone doing a '77 Bronco restoration, the after market offers only dissimilar tanks for other model years. The fact that the aftermarket has chosen not to provide accurate reproduction tanks for '77 restorations doesn't mean the originals are of little value. It's just the opposite, the value of quality restorations is increasing because so many owners are encouraged to turn their classic into something else.
I'm pretty well versed in original restorations
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You are correct in that there is no reproduciton of the 77 main tank (there is an aftermarket skid plate for the OE tank, not 100% correct, but looks and works well). Sure, to the right guy at the right time they could be worth a preminum, but the market for them is also much smaller.
My point was that most Bronco owners are not doing concourse originals as some of us have done, but would actually prefer a larger capacity tank as an upgrade for extensed range. Therefore, there are plenty of orignal used tanks on the market for the small subset of purists who need one to replace a damaged one on their restoratrion (cracked or?, as these obviously do not rust out and rarely need replaced).
If they were worth all that, I'd have sold out of them long ago, but I often drag them back from major shows with lots of Bronco people attending like the Super Celebration and the Carlisle All-Ford Nationals.
I've placed more 77 main tanks back on the pallet rack after the shows than I have sold, so this is from my personal experiance selling them at $75 for many years. If you know of bunch of guys with cash who are clamoring for them, please send them my way! ;D