I've looked and looked....
 

This is dumb, but i cant find out if my truck has a 17 gallon tank, or 19 gallon. Anyway to tell? Its an 84 F150, single cab...think the base model, L300 with 4 spd manual, the tank is what i'm thinking is side mounted, 17 or 19? Looked up for a replacement and there were two...17 and 19. How much was normal/stock?

Not sure about an 84 but on the later models the front tank size varies with wheelbase/bed length. on the 93-96 models a 6.5' bed gets a 16.3 gallon front tank and an 8' bed has a 19 gallon one. Yours should be similar.