I don't know about your 73, but I can tell you that new tanks for my 70 Bonneville convertible are not available. Many guys inquire about tanks for big cars our this era and they aren't available. If you own a A body GTO or lemans, everything is available and reproduced. There may be some parts from other models or years that could be used. I also have a 74 Grand Am. They don't make a sending unit for it, but they substitute one from a 74 Lemans. Only problem is the Lemans is a 21.8 gallon tank and the Grand Am is a 25 gallon tank, so the lemans unit probably shows empty when 4 or 5 gallons are still there, and the pickup tube isn't long enough to reach the bottom of the tank, so those extra gallons are not able to be used. My solution was to send my sending unit to be rebuilt, and it now is better than brand new and works perfectly. Tanks can also be repaired if needed. Some suggest a good radiator shop that can clean and solder any leaks. I sent my sending unit to Bob's Speedometer Service
www.bobsspeedometer.com who does speedometers, instruments sending units etc. 800-592-9673