Bob...
I still have to differ there, seen plenty of ssb 77 and a few 78 trucks with an in cab tank that was auxiliary.
Just to blow your mind a little, and I will have to look up the code, but there is a code that is for a special tank configuration in cab only, not in frame, and was on 76 and 77 d100 trucks.. have a 76 like this in my yard...