If you're going to go through the effort, there's a couple things to keep in mind.
1) don't buy a new tank from the dealership...they're well over $1k!
2) change the tank when you're low on fuel...it'll make it easier
3) go a head and change the fuel pump while you're doing it...2 birds...1 stone
4) when you get the tank from a salvage yard, make sure to get the straps. I think the passenger side is the different one, but grab both to be safe.
5) search for a '93 and up tank as the '92 tanks had some problems with the baffles
6) be ready to do your exhaust...the old exhaust won't fit once you put in the newer tank.
I don't have a clue how much it will run you. I haven't done it, but did look into it. I'm planning on keeping my tank and going with a single 3" around it. That will flow enough for my requirements, with greatly reduced backpressure. And it is much cheaper. Just keep in mind if you were planning on deleting the resonator...many put it back in. The drone is pretty bad at around 2k-2500 rpm. I'm not sure what a dual inlet/outlet resonator runs, but I'm going to guess in the $150 range...New pump...$100~Exhaust shop... that depends greatly on where you live...and the fuel tank also varies greatly depending on location. Call around to a few salvage yards and ask what the price would be for the tank. Specify '93 and up T-bird/Cougar/Mark VIII (Lincoln). You may have to actually go year by year like I do with the morons around here, but they'll be able to find a price for you.