I also have a 72 C 10 that has had damn near everything replaced on it now. I never lowered mine, but I think I can lend some help with your dilema.
But first off, is is a coil or leaf rear on yours?
If it's a leaf rear, then you will need a flip shackle kit to mount your diff on top of your leafs instead of underneath. Along with this you will need to C notch the frame to get 5" of lowering plus you need to install an adjustable pan hard bar.
If it's coil, you can buy 5" lowering coils to lower it. You will still need the adjustable panhard for this as well.

On the front, you will need drop coils and drop spindles. I believe the largest drop coils you can get for the front is 3" and then you will need a pair of 2" drop spindles.
You can find these parts at www.gmcpauls.com , or www.classicheartbeat.com . But seeing how you are in Philly, GMC Pauls would be closer. Early Classic Enterprises will have them as well.

Hope this helps