okay, so i've already picked out my lower. I'm going with a mega stipped, and a chip mccormick trigger set.

So now i'm working on the upper. I think i'm going to go with the Ameetec Arms - Side Charging Flattop Upper Receiver - Complete, rock river stainless 16" preban barrel, and rock river quad rail system. Now then, here's my question:

The rock river stainless comes with the front sight housing, barrel extension, handguard cap and barrel nut. So I am going to need a gas tube, Barrel indexing pin, delta ring, weld spring, hand guard swap ring, and what else? what the heck is a gas block for?

and I'm going with the jdtactical chrome bolt and carrier. where do i pick up the firing pin from?

Gas block is a replacement for the front sight base. If you cut off the top half of the standard FSB, you'd get the functionality of a gas block. Some gas blocks are lower profile and some have a rail base so you can mount a QD front sight.

You can order a firing pin from most manufacturers and dealers.  Try any of the ABC's like Bushy or dealers like Adco.

You shouldn't need a barrel indexing pin. It will already be installed on the barrel when you get it.