Man that thing is HUGE!! I am sure you already plan on this, but just wanted to say it any way. GUSSET EVERYTHING! Tube work looks cool and is great, but if you don't have access to a bender and stuff, use plate and lots of it. The front shackle angle is not going to allow for any compression of the spring. I'd move them back a good 5-6". Being a rear shackle, and a long one at that, under hard braking the axle is going to roll under when the vehicle weight shifts forward.
I can't wait to see it on IROC's, they are a sweet tire. Dude the unibody looks to be in good shape, I weld to mine when doing SAS's, with good result's. Keep up the good work. And by UP I mean I hope you have a ladder.