the 4bt is awesome. It would make an excellent swap. I saw one in a full size 69 f100 and a classic wagoneer.. Both HEAVY and both could squeeze 30 mpg. In the box trucks they are coupled to a GM TH400 so the conversion would really just require rear crossmember and ujoin/driveshaft work. The other SUPER COOL part is that they are completely mechanical. No wiring except to glow plugs! The only down side is the cost of entry. Good used pullouts are usually 2500+.