I know that it could be several things such as the 02 sensors, the fuel injection, the exhaust gaskets . where do I start?

The cat converter is melting down due to too much fuel in the exhaust. Do not replace your cat until you have solved the rich condition or your investment in the new catalytic converter will be in danger. Many things can cause excessive fuel in the exhaust but usually , if it melted the cat , you should be looking for cylinder missfires ( complete and partial). If you want, Street Tech Auto Care will pull the codes for free for you. We can advise you better with that info. If you want us to diagnose it further we can do that as well. Please do call or come by

It's 24 + Years old .. That 'just might' be part of the problem!

24 years and not really much information to go on......Typically (in my experience) a 'rich' running condition will cause converter failure possibly in the way you describe. Might want to investigate what shop(s) in your area have a scanner that can read live data , so it can be determined WHY the converter(s) failed, before buying lots of unnecessary parts. You might even be able to buy a scan tool that will work (VETRONICS I believe?) , like a GM 'tech1' scanner . then you would just need to know how to interpret the readings.