A few other things you may want to consider:

What altermator are you runnning (1 wire, 4 wire)? They wire up differently

Are you running an electric choke? you need a keyed on 12v for that.

HEI is a snap. you need a keyed on 12v, it will connect to the rear terminal on the distributor cap, as it is installed. the one in the front is for your tach.

My opinion, cut out the wires for the electric sending units and go with mechanical guages.

Electric fan?

Be sure to ground everything properly, heavy welding wire works great for battery cables, alternators, and grounds.

Hope that helps some.