Here's a straightforward write-up on how to with the 518 to automatically shift the OD and lockup.

Wiring Link

I've done this in my '71 D100 and the wife's LRE. Both ran the Power Tour with no problems. There's an overpriced kit with the parts available. I bough adjustable switches (don't remember the p/n), ran a length of hydraulic hose into the cab, and adjusted the switches to shift into OD when it "felt" right before mounting them to the trans.

In a truck, the only mod required was a custom rear trans mount, but the OD unit sits right where the rear mount on the 727 falls, and on a car that's in the middle of the T-bar crossmember, as noted. Make sure your mod to that crossmember is up to the forces involved...