Kenny, I would trade the 2003 HD rear axle for a GM 9-1/2" semi-floater like the one I used. GM used the 9-1/2" rear axle in these heavy-half ton and light-3/4 ton trucks back in the day, and they are available in 6 lug to fit the half ton wheels. I haven't broken the rear axle in my truck, even with 600+ rear wheel dyno horsepower. The 2003 HD axle is really too heavy for the chassis and the mismatch in wheel lugs would complicate things. You can read more about the rear axle I used here:
http://www.thedieselpage.com/reviews/randys.htm
I recommend keeping the original 1995 and newer dash, just modify the cluster mount to accept the 2003 cluster. That will greatly uncomplicated the conversion. Though, you should fit the 2003 steering column, which will give you the correct cable shift mechanism for the Allison and all of the electrical will be plug-n-play. In the 1994 model year vehicles, It's a toss-up whether to change the entire dash... I'd look hard at finding a way to keep your original dash. The dash conversion in my truck was as or even more difficult in some ways than the engine conversion - took nearly as long too. There are ways to do a conversion while keeping your original dash cluster. There's a motorhome conversion discussion here in this forum about just such a thing.
You can read more about my intercooler installation here:
http://www.thedieselpageforums.com/t...ad.php?t=28375
Jim