Since it is in a garage closet, I would not bother to patch the drywall. Get a piece of 1/2 inch plywood and cut it several inches larger than the hole. Put a few screws into the studs. If it ever leaks again, you have an "access panel".
Don't now if this helps but I frequent the Model T forum. There are tons of information on cleaning out Model T tanks with an aggregate or evap-o-rust. One guy said :
"I have used Lacquer thinner on several gas tanks with good results. Leave it in the tank for several days and shake the tank...
I read if you put a Tums tablet in the hole before you plant your tomatoes you won't have any problems with Blossom End Rot. Haven't tried it yet as my plants are already in the ground. Gonna try it with my late tomatoes. Usually, about 1 Tbs per plant of Epsom's Salt works if they start...