So that is a 20 gallon tank.
If you just want to replace the tank and don't care if its model year specific. Then your hunt is going to be much easier.

Just look for 12 x 12 x 33 or even longer in length and you may find what your looking for. Or 20 gallon search and just compare dimensions.
The good news is you have plenty of room under for a different tank that can be close to that size and not necessary exact.

I could not find a good GM fit so just so happened my model year has a ford tank with dimensions I was looking for. Tanks a tank and I am not doing a true restoration.

Depending on budget you can find some nice ones online in 20 gallon range aluminum and other items. there are guys who will even custom build you one.

I found a web site that had tanks with dimensions easy to search but now I cannot find which site it is. Soon as I find it i will post it.