'68 Dodge Dart: Two openings in trans tunnel 4-speed manual

You are correct, that it is a factory 4 speed car.

Here is a picture of what the console mounted tach wiring looks like.

Console Tach wiring.jpg
Green wire goes to the negative side of the coil.
Orange wire is for the light bulb power in the tach.
Violet wire goes to the positive 12v accessory.
Black wire goes to the ground.

I'm doing this from memory so if someone thinks I'm incorrect then pipe in...