1974 Blazer 4x4 (need info on part)

[1968ratrod]

I have a 74 Blazer and the fuel sending unit is bad and I can't seem to find the info that I need to get the correct one. It shows that there is 2 options one being 25 gal. and the other being a 31 gal. tank. I dropped the tank and pulled the sender out but the only marks seem to be 113 4 on the electrical part of the sender down in the tank. How do I know for sure what size tank that I have for sure. Any ideas ? Thanks in advance.......