help setting up my 8 3/4!!!

Hey Fabo members, I'm again in need of your help! My goal is to take my Dad on a trip down Rt.66. I'm building an 8 3/4 w/wilwood disc brakes. My question is what gears can I use in my set up? I'd like to get some mpg's!:) Any and all help is always greatly appreciated!!:) here is my set up:

I have a rebuilt 340 with a rebuilt 904 transmission. The engine has the following:
1973 block bored .030” over
10.5 to 1 TRW forged pistons w/moly rings, rods resized and bushed w/floating wrist pins and new locks
Pistons and rods balanced
ARP bolts crank polished
Clevite 77 rod, crank, and cam bearings
Windage tray
Dynagear tru-roller timing chain
Crane cam 284/480 degreed
J heads, 3 angle valve job
Comp Cams locks and push rods
Mopar perf. valve springs and adj rocker arms
Edelbrock Torker II intake
Edelbrock thunder series 500cfm carb
Mopar perf. black alum. valve covers. Electronic ign. distr.
Carter fuel pump
Mellings oil pump

The 904 transmission had also been rebuilt with performance parts. It has a Dynamic 3000 stall torque converter, reverse manual valve body, and an Cheetah SCS shifter with console.