Had a nice phone chat with the old Volvo mechanic in town who has a few good overdrive transmissions stashed in his huge parts heap. He'd offered to install one in my car a couple of years ago but I was spending money working on the body and some other more pressing mechanical issues.

Today he said he could certainly install an overdrive in my car while I am out of town in a few weeks. He has the shortened drive shafts ready to go. He mentioned he'd like to install the larger, older model of overdrive since they were more reliable and easy to fix. Is that the D-type? Then he mentioned that he may have to do something to the tunnel. I understood that I would need a shortened drive shaft but I didn't know tunnel modification would be part of the equation.

Since he has both types of overdrives, should I ask for the J-type? What are the primary differences.

My car still has the original B-18 and it runs great on the '75 240 electronic ignition this mechanic installed last year. But I would love to have lower revs and better fuel economy on my highway jaunts. I'll also get a new tranny cover that will solve my wild shifter bounce-a-bounce.

Thanks, as always, for yoru knowledgeable insights.