74 Swinger brake CLUNK!
a whole bunch to unpack here...
first question: what's your level of mechanical ability? are you able to ascertain when something is borked by looking at it, or putting your hands on it? can you read and interpret a FSM? do you have tools and a place to wrench?
I ask this because I want to know if are you attempting to do this work yourself or are you taking our suggestions to a shop and feeding them 3rd hand information?
anyway, without your hands on the car or pictures or a detailed correct description of the problem, everything is kind of conjecture.
best I can say is jack it up, drop it in gear and see what's cracking.
also, your fuel consumption is unlikely to be correlated to the problem you're having. if the drum was dragging badly enough to make the fuel gauge move it would be making one hell of a racket and you'd smell the cooked friction material. it would also be blazing hot.