You definitely need the lockup. On long trips in overdrive, the trans will overheat and convert itself into a boat anchor. The other critical adjustment is in the TV cable.

http:\\www.bowtieoverdrives.com

A little wiring in the trans, and a switch in the brake line, and you've got it. You can also add a manual switch to lock it up in any gear.

Call them for an application. They also have some interesting reading on the site.