The cost of upgrading from a 727 transmission to an overdrive unit is well worth the investment. The additional costs of converting to overdrive will pay for themselves in fuel savings over time. In addition to fuel savings, drivability of your Mopar will greatly increase. The reduced noise level from your engine revving makes long-distance driving much more comfortable. And, sustained driving at a lower RPMs puts less wear on your engine.

My Cuda has a Vortech supercharged 408 attached to a non-lockup a 518 transmission. It was one of the best upgrades I did to the vehicle. Because of the transmission, I drive the vehicle a lot more and I put regular highway miles on the car. On the highway, I get about 17 miles to the gallon. Years ago, I had a 1986 Mustang with a Vortech supercharged 351 and a five speed. At best, I could only get about 18.5 miles per gallon. My Mustang and of the Cuda weigh about the same and they both share the same fuel injection system. I would imagine that if I had a lockup a 518, mileage would be a little better.

Based on my experience and the enjoyment I get out of driving my Mopar anywhere and everywhere, my vote is that you spend the extra money converting overdrive. In fact, the conversion to overdrive made more of a difference to gas mileage than switching to fuel injection.