I think you are right, but i thought an older tbird tank might fit.
yeah, but again, you can't do that especially with the older tbird, by which i assume you mean the triple, because those weren't even injected. So even if it were possible yo use a different tank from a injected bike of a completely different model, the old tbird has NO fuel pump. So it's impossible. As i said about thos even if you tried a bonnie or speedmaster tank, sure they have fuel pumps in them, but they are likely a totally different one that even if it mounted the same (VERY unlikely) they likely would not work with the Tbird's ECU
In short, you're barking up a tree thats not even there. It just ain't gonna happen, at least not without major modification and major $.