On the 64 C-10 rear coils were standard and leafs were optional. Its opposite on the GMCs if I'm not mistaken. For a "6 rear drop on coils you may want to do both blocks and springs. If you are on a tight budget you can cut the rear coils a little and buy/make blocks and U-Bolts. The Block/bolt I used on my '69 Burb I had originally made for my '61 C10. I can send you pics of the rear set-up if you’d like.
Up front for a 4" drop you will have to include drop spindles. 2.5" spindles and 1.5" cut out of the coils would work.
Ideally though, short of going way custom, aftermarket drop springs and shocks should be used with the blocks (or flipped leafs) and spindles.
