I have been through this scenario. I sold my orange shortbox for the same reason. horrible mileage. may have got 7 mpg. tuned well. just a pig on fuel.
my opinion...probably wont be popular. low blocks are not for trucks. especially a 160-ish hp 400. pitch it put a 440 in it or a good magnum motor.

I will never run a stock low block ever again unless its for a resto.