This from sticky in Transmission & rearend section; How to calculate rear end ratio: http://www.slantsix.org/forum/viewtopic.php?t=36418

These engines respond to more carburation such as a super six 2v set up, and larger diameter exhaust say 2.25" this would be without simplest mod without pulling the head. As Josh said recurve the distributor's timing impute.

These trucks came out of the factory calibrated for emissions & fuel economy, the problem of the day was performance and drivability had to take a back seat. Every thing built from 1973 until fuel injection became industry standard by mid eighty's had poor performance and often drivability problems; 73-75 being the crappiest of the period (pre cat). Once catalytic converters were fitted some of the performance and drivability returned, but still performance and economy mostly stunk.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

It is a truck,

My 82 D150, four speed, no OD, 3.5 rear, does great between 55 and 60 mph, I can pass if needed, but once I hit 65, the engine sounds and feels over revved, it does not handle well, and my mpg goes way down hill.

These were not built for speed. But they sure make a great work truck..

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Slant power!!!
At least I try...