That kit, in my opinion, is NOT a good selection for trucks. It uses CAR, (Chevelle), disks and 10 inch rotors that will not result in much, if any, improvement in braking over stock drum brakes.
A better choice is the kit that comes from Early Classic Ent and uses TRUCK components such as 12 inch rotors and comes with a choice of stock or dropped spindles.
So, I don't really have an answer for you with that kit you selected. The best option for lowering for you will be 2 inch drop springs. 2 inches would be the max I'd advise to keep your front end alignment guy friends with you and to keep the fillings from rattling out of your teeth.
I would avoid trying to make changes to the crossmember in order to get that kit to work the way you want.
Sorry, I know it's not the answer you would want, but I'd see if CCP would take that kit back and get one from ECE..
dv