I had your tach wiring figured out by the time you posted. Most guys do it this way but there are other ways to do it.
Here is a diagram of a way to wire up an updated electrical system using a distribution block (junction block) with fuse wires for all the electrical loads.
It also has relays for the headlights and the cooling fans.
You could use relays for the seat heaters as well, in fact you should since you're using the IGN UNfused terminals for the HEI distributor. You want as much voltage as possible to go to the distributor as close to 12 volts as you can get.
Ignore the auxilary battery but note the 8 gauge wire feed from the alternator to handle the increased current you may draw.
