I have used the Gearvendors "Over/UnderDrive" on a 1991 Dodge Cummins Diesel.
It is hell for stout.
The ole truck had 4.10 gears and a Getrag 5 speed.
It got 22 mpg prior to the overdrive install and it got 25 mpg after.
The unit that you are asking about also lets you split some of your gears which lets you stay right in the engine's torque curve.
I paid $800.00 in 1992 for mine and did the install myself, except for the driveline work.
The only concern I would have is that on an old truck, the OD unit may allow you to put too much strain on the old gearbox. However that m,ay not even be an issue in your case.
They are expensive units but they work as advertised.
Good luck.
