Interesting, I have this same project going on with a K5...
I was planning on using period 2500 or 3500 truck mc/booster and using a vacuum pump, comments on this? Seemed simpler than the hydroboost...
The 2" body lift you already have is what I was planning, (rough) measurements off my 05 dmax and an old k5 body I have seem to indiciate everything will fit without too much hacking. If anything a bit of tunnel fab and things should be okay?
My main concern is finding a frame solution for the k5, I want a 1-ton frame but it appears none of the 1-ton or 3/4-ton frames will fit the k5 decently (based on where the hump is for the rear axle... did they even make extended cab 3/4 or 1-tons in that era?) I assume you're using the 3/4 burb that is for sale for the last time again :P
Good luck on getting the drivetrain, this should be an awesome project for both of us! I absolutely love the duramax. Keep us updated.