Hooking up factory tach harness-78 Chevy

[6772fan]

I bought a factory tach from BigJake97 on the truck board. It's from a 79 Blazer and I'm putting it in my 78 Chevy truck. It came with the factory tach harness and here's my questions:
The 3 prong connector plugs into the back of the tach, then there are 2 other wires that I don't know where they connect. One is oblong in shape with a male clip on one end and female on the other end. The other one runs about the same length and it looks like it connects to the fuse block, but can't tell where to connect it to the fuse block. I've already drilled a 3/4 inch hole in the firewall and connected the other end to the distributor. Also, the best I can tell they appear to have the same connections at the back of the guage cluster, since I have factory guages already, is it a plug and play or do I need to re-pin any of the connectors at the back of the gauge cluster? Any help on this is appreciated. Thanks, Lane.