Newbie needs help with lowering

[fightermatt]

Hey everyone, I'm new to the board and I had some questions about lowering my new 77 C10. This is my first project so when you see me screw something up, just let me know. My fist question is that I've heard people talk about backspacing, what is that, and my second question is that I've been looking at a 4-6 lowering kit, but I just came across a 5-5 kit for less money but my buddy told me that a 4-6 would be more street friendly because this is a daily driver. Any advice.

P.S. These are the only pics I have right now, I'll start a build thread once I get going.