If we are talking about where things are made, it should be known that cummins blocks (at least the pre-2003 5.9) were cast in Brazil(dreaded 53 blocks
) and Mexico.
I also believe that GM bought out Isuzu's interest in Duramax Ltd. making the Duramax 100% GM, but I've been wrong before.
Which engine are you using, LB7, LLY, or LBZ? I see you have a 2003 2500, but I cant imagine you would tear apart a truck that nice, would you? If you did I guess you could always put a 6.0 Vortec back under the hood, they run pretty good for burning gasoline. I have plans for using an LB7 in another truck but I wanted to build something I could have a lot of fun with, without the electronics or automatic trans.
The frame is looking good, and this is turning out to be a pretty good discussion.
