Wellllll there are sevral routes you can take
You say the truck is being built for mostly mud, what size of engine are you droping in? Thoes big tires you want are gonna punnish your front 1/2ton axel...I hear ujoints snapping
Money Pit =
Front D60
Rear 14FF
454 w/SM465 or TH400 for you slush box lovers
I would keep the lift low and cut sheet metal, a solid well built body lift will help too(somepeople hate bodylifts).
Take measurements from eye to eye and eye to centerbolt and shop that way instead of by year. Also keep in mind that the rear of Blazer is much heavier than the rear of a PickUp. MOST Pre-made spring packs are made using PickUp weight specs NOT Blazers. That is why you see saggy butt Blazers when people install say a 4" front and 4" rear = Saggy Butt.
I run 4" front & 5.5" rear springs and I sit level when the hardtop was on, without you can see a slight stinkbug..but that is better than a saggy butt
Burt