Leaving the original 1961, 235 CID slant-six, I ripped out the 3-spd, and 4:56 rear end, and replaced with a M4 overdrive and 3:50 rear axle out a 1977 model Dodge D100 pickup.
Top speed went from 50-mph to 95-mph ( with aero pkg ).
Fuel economy went from 11-mpg @ 50-mph, to 21.5-mpg @ 65-mph.
Lower speed reduced fuel economy.
With zero aero mods, It gained 5-mpg.
I had to fabricate a custom hydraulic slave cylinder mount, for the 'modern' clutch ( mocked up in plywood and subbed out to a local machine shop ).
Also a custom driveshaft due to length differences.
And I had to modify for the different foot-operated-cable rear brake emergency brakes, from the original central driveshaft dash-mounted hand-parking brake.
I paid $150 for the trans, and $ 150 for the rear end at a nearby wrecking lot. Pulled them myself and carried them to the truck in a Radio Flyer wagon.
Chrysler Corp didn't waste $ on R & D.
