A Cummins 6BT (5.9) in a '42 1-1/2 ton dually pickup is my project. I don't have the engine mounted yet, but all of the measurements I've taken show it will be a very tight fit length-wise, but it *will* fit --- without having to modify the firewall. The only real modification is that there will be no room for the mechanical fan. My idea is to use an electric fan running in reverse (pushing air, rather than pulling air) in front of the radiator. Width-wise, I believe it is better to use an alternator mounting bracket (and maybe AC Compressor bracket, too, but not sure yet) from a medium-duty application rather than the Dodge pickup version because the nose of the engine bay is quite narrow.
I am copying many of Grigg's ideas for the drivetrain using P30 and 3500HD parts.
Grigg, you mentioned you had not yet seen an *outstanding* 6BT conversion. And judging from your project so far, I'm sure your definition of *outstanding* may be a little higher than mine. Take a look at this recent Diamond T on Ebay: I know it's not GM sheet metal, but it does use a P30 chassis. In my opinion, it looks pretty close to outstanding, except that the overall stance maybe looks a little low for my taste. Maybe he just needs more air in the air bags.
Bob
Ebay #120393445886
http://cgi.ebay.com/ebaymotors/1949-Diamond-T-tow-truck-Biggest-Street-Rod-in-town_W0QQitemZ120393445886QQcmdZViewItemQQptZUS_Cars_Trucks?hash=item120393445886&_trksid=p4506.c0.m245&_trkparms=65%3A13%7C39%3A1%7C240%3A1318
