I started putting a Cummins 5.9L (BT6) in my 60 Apache 30 Panel a while ago. Unfortunately I'm out of work so its on hold for now but here is a link to what I've done so far.
http://67-72chevytrucks.com/vboard/s...d.php?t=523009
It is taking a LOT of modification to make it fit right. I'm pushing the engine back and loosing some cab space. Lots of firewall modification. However I have seen the same conversion where the owner stretched the front out another 6 inches but that means frame and fender extensions.
However, from what I've learned so far, a BT4 Cummins would likely fit really nicely with a lot less modification. I also have heard that there is someone here that is doing a Duramax conversion. However I believe that the Duramax is even bigger than my BT6.
Hope this helps