The crossmember wouldn't be required for your truck at all. It is required with low lifts (2-3") and setups with lots of travel (i.e. coilovers). The crossmembers are notorious for cracking out but not necessary for crossover for your truck.
Dude, HUUUUUUUUGE fan of that K5!!! I did my Bronk up like a goofy ultra-4 looking thing, then saw your Blazer and WISHED I had went that route. Now, my brother-in-law just bought a K5 and I've talked him into stripping it and building one like yours. Hit me back, man, I'd like to get some details so we can get it that way! -Mike