personal rule of thumb - do the suspension before bodywork. that way all your cutting and making room for other stuff (unexpected stuff) is done and you can go from there.
the difference in the viair 450 and 380 is that the 450 is rated to 150 psi and the 380 is to 200. that pressure switch might not be a good idea if you are running a pair of 450's. in general the more the merrier when dealing with compressors. nobody likes a compressor that runs forever trying to fill up a big tank.
cantilever means pivot - when you have the bag pushing up on say a bar, and the bar pivots in the center and pushes down on the axle, like a teeter-totter (sp?) to lift the truck. that's the best way i can describe it. you can mount the bag and pivot wherever to best accomidate you're lift and drop.
this is a leverage setup - not cantilever (contrary to some)
this is cantilever - see the pivot point above the bag?
imo cantilever is not necessary. its cool, but a nice leverage setup would be easier for you to build.
Retro 83 said
the cnotch is ONLY there to provide upward clearance for the axle to travel. not for axle wrap or anything else. just to provide clearance.
don't forget to multiply the cost of the valves times 8 and the bags times 4! that might make a kit look a little more in the ball park.
oh, and gauges. you can get by with 2 dual needle gauges, from airlift or airride, smc? i don't know who all makes them, but you should want 2 dual needle, one needle for each bag. you can get one more single needle gauge for the tank, but if you are running a pressure switch, it would be redundant...