You'll pick up some MPG's with the OD, so I'd say its worth the trouble. That trans isn't exactly a bolt in deal though, it's longer than your 3 speed so the drive shaft needs to be shortened. The shifter locations on the truck OD trans doesn't work in an A body so you'll need to fab up a bracket to mount the shifter in the right location. Here's a link to the shifter bracket drawing: http://www.bigblockdart.com/techpages/abconv.shtml
You'll also have to trim part of the forward shifter mount off the trans to clear the cross member.
Like I said it's not exactly a bolt in deal but it's worth it in the end, it'll pay for itself in fuel savings.
