Joel- better to ask than to fry something!! Good question!
The tach lead is the brown wire, and if you put on a female spade connector end onto the wire, and plug it into the tach lead on the HEI distibutor it will work as original.
Now, for more info than you asked for, be sure not to just use the original coil wire for the HEI power lead. This is the wire with the fabric cover on it. This is a special ni-chrome resistane wire to limit voltage to the coil. It will not supply enough voltage for the HEI to work right. You must either run a seperate lead from the ignition switch, or change the wire in the harness. Either way use at least 12 gauge wire. If you need more info, be sure to ask!!