[Big Kev-O]

It all depend on if you have a leaf spring or trailing arm truck and how low you want to go. You can use C10 drop springs you will lower the hauling & towing capacity unless you go with air bags. If you want to get real low you are going to need a C-notch which is not recommend if you plan on towing. There are C-notches that claim to be capable of towing but I personally wouldn't tow with one without welding in some serious bracing.

I dropped my 68 C20 using 2.5" drop spindles, air bags & a heavy Duty C-notch from CPP. All of the drop spindles require you to upgrade to disc brakes. You need spindles designed for 63-70 trucks in order to reuse you existing ball joints & tie rods. 71-72 trucks have joints & rods. You can use 71-72 spindles but you will need to change your ball joints and use you outter tie rods in the inside position with 71-72 outter tie rods and a special tie rod adapter.

There are quite a few threads about lowering C20s on this site. All your questions have already been answered just need to use the search bar.

If you have any more question I probably know the answer. Good luck!