Josh, If you use a harness specifed with tach (67 on up) you will lose the use of you ammeter gauge and your oil pressure gauge. Those harnesses arranged for "with tach" convert those circuits to whats commonly referred to as "idiot lights".
The underdash harness, firewall to headlight harness, and alternator harness are specific to cars with tach or without tach. The differences are small but very significant
So I suggest you purchase harnesses for a car without tach IF you are going to install an aftermarket tach. Most all aftermarket tachs come with a wiring diagram (as manitario indicated) on how to connect it using spade tongue terminal connections.
None of the after market tachs operate like ford "Faira" type tach which is a series type tach.
Just pick one out and we'll help you wire it in.