new guy with a 69 c10 and questions

[nasgue]

i have a 69 c10 longbed. i bought it from my uncle who says he bought it from the 2nd owner and he was told it had a 327. I've heard of mid year trucks and i think that's what i have. Anyway, i'm looking to get pointed in the right direction for help. i've searched the forums but i feel like i'm missing something somewhere.

I have a speedometer and gas gauges on my dash panel. i wanted to upgrade to include a water temp, oil and battery, gauge but i'm told i have to change out wire harnesses. Is this true?

Also, does anyone have a diagram that show's what is supposed to be positive/negative in the fuse box?