Any over drive trans will produce about the same effect as the Gear Venders unit at a much lower price. The only advantage the GV unit has is it can handle more power. The 727 version of the Mopar overdrive automatic can be as stout as your current 727 or most of the Mopar OD 4, 5 or 6 speeds will work at probably 1/2 the cost of the GV unit, even after converting them to fit your big block.

My 93 Dakota 4x4 pickup has a Magnum V6, 500 auto od, 3:55 8 1/4 rear and 235-75-15 tires. It will roll down the road with the od off (switch on the dash) at about 2600 at 60 mph. Turn the od on, and the rpm drops to about 2100, and when the converter locks up, the rpm drops to about 1800 at 60 mph. The V 6 doesn't have enough balls to pull a long hill at a steady 60 (it does better at 70) without unlocking the lockup converter, and if its a really long hill, it might drop out of od. Out of od, I can gain speed going up nearly any hill. A V8 would probably pull the hills without issues. On the highway, my 4x4 Dakota gets around 20 mpg at 70 mph. Gene