I don't know if they will interchange (I do not recall that they will), but some measurements will answer that one. In answer to your second comment on pricing - it's mostly based on costs and production numbers. All tanks have a labor and material cost to make them that are fairly similar, but the pressing dies can additionally cost $20,000 dollars or more to make for each model. If you're making something common (Mustang) the cost to make 1000 tanks will be $20 each, just for the pressing dies. If you can only sell 200 tanks for a less common model, the price shoots to $100 each (5x as much). Considering the additional cost of materials, assembly, stocking, shipping, etc., and that every stop along the way to you has to make some money to do it, the prices begin to sound more reasonable.
Don't forget you may have the option of repairing or stripping and relining your original tank. Check the phone book, and many radiator shops do this service as well.
PS: I just looked-up one for your '69 Fairlane, and at under $200 is pretty reasonable. Add about $40 for bulky shipping rates and you're done under $250 for a new tank. Do searches for an F43A tank.