Just got an email from my friend this morning:
"...most of the trannies are the same, but he needs to find out the make and model of the transmission. If it is one of the old New Process units or a medium duty Spicer, he is all set. They are all pretty standard. Usually, the worst thing he would have to do is have the flywheel remachined for the center bearing. (Meaning a pilot shaft of a different diameter) This would not be the norm. These units were, for the most part, made to interchange. He might even be able to get away with just a new bearing pressed into the flywheel. My bet would be it will fit right in if he uses the housing from the diesel.
Make sure you tell him that the tranny he is getting might be a 5th direct and the one in the diesel might be a 5th over, which would reduce or increase his top end speed...
...that engine is not as bad as some say it is. I would not put it against a DT466 or a B series, but the numbers say it is right in the same line as the Navistar turbo 7.3's for CPM in a 1/4 cycle unit...
<<CPM 1/4 cycle is his slang for "cost per mile" in a truck that has a predetermined life of a quarter million miles, usually the lighter trucks like a 20-24' single axle box truck>>
...the real bad engine from GM was the 6.2/6.5 in the light trucks when they went to the fly by wire pump. In the van fleets, any of these were a nightmare. With the older 6.2/6.5 the head gaskets now and then would blow. Usually not terminal and not too bad to fix. The pumps on the bigger version like he had had one bad year, but I cannot think of which year. Either way, if the pump fails, a new ring kit will fix it for life.
If I were him, I would find one of the 6 speed Spicers with a highway rear like you are planning and make the swap. They are cheap and plentiful.
PS You are a *&^%ing hippie. School Bus boy!!
I hope this helps you.
This guy is a friend of mine I used to work with who manages a fleet of 3600 units. Sounds like you might be in business.