NV4500, SBC, 4:10 and MPG

[Aruba1]

Currently I have a 1972 C20 with a 1989 C3500 front end and a 4:10 rear end. The tires are 225/70/19.5 (dually). The motor is a 383 SBC bored over 30 with a holley carb. It has a low cam (not sure of the numbers), stock manifold and dual muffler.

Now I am running a TH350 but my plan is to do a lot of heavy towing. The tranny is excellent shape now so I want to go ahead and install a NV4500 gearbox before I mess it up. What sort of mileage will I be seeing with this gearbox? I am no longer a pedal to the metal person .

Thanks for your help.