[67c10rustbuket]

generally everyone uses a (firestone) 2600# bags front and rear on a full size chevy, and firestone 2500# on front of an s10 or other mini w/ 2600# on the rear . some use 2500# all the way around on a mini so they can carry 1 extra bag instead of 2 different ones.
as for your truck i would use a 2600# firestone (re7 equivalent i think) for front and rear. on the front you want a bag that will put the suspension at full lift without stretching the bag. if you use a bag that only gives you 6" of lift without locking the front suspension up you will stretch the outboard side of the bag. example if you use a 2500# bag on the front of a full size truck you will get less lift but it will require higher pressure and the outboard side of the bag will be stretched and eventually rip or blow. if you think about it the inner side of the bag does not lift as far as the outer because its closer to the pivot point. and if you get a smaller bag that only lifts the front to 6" it will ride like your sitting on a brick at 6". you have to have a little extra up/down travel from your desired height to make a comfortable ride.
a smaller bag requires more pressure to lift than a larger bag. more volume uses less pressure, . in a full size truck your pressure to achieve desired ride height should be around 90-100 psi front and 40-50 psi rear depending on weight . assuming thats a small block, a big block would require more like 120+ psi to lift the front. also at full drop the bags should be fully compressed (3" i think between mounts cups whatever) if the bags are not fully compressed this will cause you to use excess pressure to lift the truck.
weight , bag volume, lift angle/position, all change the general psi of lift.
if you only want 6ish" in the rear it would be best to use over the axle mounts rather than in the stock spring location and the bags will last longer due to them being worked in a more direct up/down motion. in the stock spring location the will be inflated on an arch where the rear side of the bag will be lifting higher than the front.
I have used upper and lower "cups" but i prefer plates on the bottom that bolt to the control arm because the recess in the control arm does not have a consistent depth due to the shape of the coil spring. there will be a lower spot where the end of the coil must be clocked into so the recess is not flat in the bottom so the cups can wabble around. not to mention that when deflated the bag will squish out the sides (like a flat tire) so its nice to have a flat surface for it to squish on instead of a sharp edge of a cup (depending on the diameter of the cup).
another thing to think about most first timers mess up is the air bag may fit into the spring pocket just fine when its new and un-inflated but when you inflate it it will not only inflate up it will stretch outward so make sure you have close to 1/2 inch clearance around the bag to spring pocket. this is a huge problem on s10s.
I think most retailers websites list the specs of the bags they are selling i know i used to check them out as far as compressed height, full lift height, suggested ride height, bag material ply, weight lifting capacity, max psi, etc..
the truck looks awesome and theres nothing wrong with a big fat Z as long as its all aligned and welded properly. nice project !