What they're going to tell you is you want to have everything in your rig you want to have as a finished rig....for the most. Some things can be figured out as far as weight goes but they need to build springs off a total weight of the vehicle. They will tell you if they build you a set of springs for your rig, then you go and put a cage, big winch bumper and winch, rear off road bumper and whatnot, it will cause sag in the springs and wear them out prematurly.
You will be expecting a say 4 inch lift or what ever you tell them you want, but once you get all the weight in the rig it's sagging by 2 or 3 inches, you will be disapointed, and they dont want that ofcourse. So they need to know a total as complete weight as near as possible, AND they need to know the weight on the front axle and rear axle, so they can build the springs spicifically for each end of the truck.
I have talked with them about this same conversation. Part of the reason I am completeing building my K5 with a 63" rear spring conversion using 88' and up 1/2 ton springs with a DIY shackle flip, and Tough Country 4 inch lift springs up front, completing the 1 ton conversion, building the cage, installing the fuel cell, getting all my Mastercraft front & rear seats cage mounted, everything done so I can weigh the truck and then custom springs can be built that will work with my truck. Your not really ready for springs till you have a complete rig.
It actually may not be Deaver springs I run, might be Alcans, not sure just yet, Deaver is local to me for the most though. Hope this helps.