I just did a repair on my 79. Pretty ez stuff. If yours is the same as mine, which I assume it is I would forget the circuit board and just wire in the tach with some wire. The tachs are clearly marked on the rear posts +, - and tach. Just wire the the + to a switched ignition source, - to ground and the other wire to the tach terminal on your distributor.
Then you don't have to worry about the circuit board and wether the rest of the rig is wired for the tach.
Here was an excellent article, helped repair my tach which was reading about 1000rpm too high.
http://howtoalmanac.com/kevin/projec...chfixtruck.htm