[Big Dave]

Lots of guys have mentioned aero cads, aerolids, wharever you call them, and make no mistake: They work.

But that TH400 slush box is killing you.

Trucks of that vintage were available with NV4500 five-speed manuals, with a GM bell housing. You should be able to find one in a good junk yard or if you intend to drive it a lot reman trannies can be had.

For really good mechanics with the right tools and some help, the job takes a long weekend.

Not only does the NV4500 gain you 2-4 MPG by itself, it opens a world of hypermiling techniques.

As someone pointed our 6.2 or 6.5 liter GM diesels are available and are bolt ups. Get the 6.5 if you can find one. Its less troublesome than the 6.2.