What about swapping the GM six into the pickup with a tall enough transmission? It may even be possible to do it for free since SBCs aren't exactly cheap. My pickup is probably just as bad if not worse than yours aerodynamically but I can still pull ~30mpg@55mph because it has a 2.4L engine. I've read/heard ~25mpg with the TBI six is possible if you keep your foot out of it with stock gearing.
