need help bad

Changed the tach board in my 1980 today. It had a burnt spot on it so figured sure that was the problem. Anyway with the tach out I tested for 12 volts and had it, also the ground was good. But then I tested for continuity from the prong behind the tach to where it connects to the filter or whatever you call it. And anyway no continuity. Now what? I really didn't want to pull the dash out or replace a whole harness. Any suggestions?????   Bummed out. Any help would be welcome

The white wire from the distributor goes both to the printed circuit board, AND to the ECM. If you checked continuity from the circuit board to the dist connection, you should have it, unless there is a break in the wire somewhere...which is not good....but you already knew that.
The first place I would look for that problem would be the fuse box itself. There are two parts of the fusebox...the inside part that houses the fuses, and the outside part under the hood. The two parts of the box, called a pass-thru at that point, are held together with a screw right in the center, on the engine side. This is also where the engine harness connects to the interior of the car. The connection there is prone to corrosion, and it's possible to have a bad connection, creating all sorts of issues.
If it were me(and it ain't), I would run a temporary jumper wire from the dist to the spot where the circuit board plugs into the tach, where the white wire goes. If the tach then works, then you'll need to start checking connections from one end to the other.
If the tack does NOT work with a jumper, try unpluging the filter. If it works with the filter unplugged, the filter is bad.

Thanks will try that maybe this afternoon. I have unhooked the filter and tried bypassing it but it still didn't work. Will try again today, and am sure you will be hearing from me. Thanks

well spent most of the day again on this tach thing!! Probably not good to do it this way but I was careful, I made up some more jumpers and with the tach out hooked one to the power clip in the dash to the power side of the board, and another from the ground clip in the dash to the ground on the tach board and ran another jumper from the signal side of the tach board and unplugged the tach wire at the distributor and put the other end of the signal jumper there and with the tach in my lap I carefully started the car and guess what? Nothing !!!! Some articles say that the tach should move just by turning the key but it doesn't do that either. I put it back together until I get some more tips or figure something else out. 

Most of the time, when you turn the engine Off, the tach doesn't go to "0"...it usually sticks up in the 1000 rpm range or higher. When you turn the key On, it goes to zero.
Has the tach EVER worked on this car?
Have you changed the ignition coil at any point?

I just got it about a month or so ago. It hasn't worked since I bought it. I have tried about everything that I can think of

It's possible that someone has changed the ignition coil, and neglected to re-insert the terminal for the tach feed in the cap. Might take the plastic top of of the distributor cap and verify that there are 3 terminals in it....

Feel a l;ittle dumb. But I did get the tach working today. Took it out for the 3rd or 4th time, to try things one more time and realized that I had left the 3 little fiber washers off. Put them on and wooooala, it works. Does about 3 grand sound about right for 65 mph ?

Sounds about correct but I have an 82 and the transmission is different that yours.  So someone else will have to confirm it for sure.

It's always something silly, ain't it? Glad you got the tach figured out! 
3000 @ 65mph is about right, depending on the diff ratio.