If you have a 518 and still want it to work like an auto, just buy an normal shift kit and use pressure/vacuum switches to engage lockup and overdrive.

Transcenter Swap Info


You tap the pressure line on the trans and 1 PSI is approximatly equil to 1 MPH. So with in this diagram the OD would kick in/out at 44 MPH and the converter would lockup at 52 MPH, a drop in vacuum would act like a kick down disengaging OD and unlocking the converter.

It should work the same with a manual valve body. I would add a switch between the 44 PSI switch and connector to manually disable overdrive.