What options for O/D behind a S/6?

My 7 + years of Slant 6 experience taught me that it gets the same mileage as a EFI 5.9 Magnum if you stay off the throttle on both. I'm building a factory computer EFI reflashed for a 408" motor hooked to a beefed up 200R4 with a lock-up torque converter and 3.91 rear end gears. I expect it will get in the high teens if not low 20 mpg at 60 - 65 mph. If I swapped in a slanty in place of the 408"r, it would do about the same.

I got 18 mpg at 70 mph with a well-worn 5.9 Magnum with a small cam and headers and a non-lock-up 727 with 3.23's going from Willow Springs to Burbank for Spring Fling. That was the best I ever got. My Slant came close, but never reached that good of mileage and it had a 904 & 2.97's or something in that range.

Bottom line, the slant ran a 19 second 1/4 mile and got slightly less mileage that a worn 5.9 Magnum that would do mid 13's in the 1/4 and still get 16 average on the highway without overdrive. I never got good gas mileage with my slant 6 for the sacrifice you give up in power when you want or need it. Do yourself a favor and swap in a junkyard EFI 5.9 Magnum, bolted up to a 727 or a 200R4 and then you will have the best of both worlds.

I know my slant liked LOTS of initial timing. I had it set at 22 initial and 32 total. The vacuum advance was hooked to manifold vacuum and it really ran at it's best when set there.