Well the instructions say its compatible with 81 but someone may have already altered your wiring harness if the wire colors were different. Maybe you missed the white stripes on either green or tan or light blue wires and confused them with the unstriped versions?
THESE ARE THE PIN SWAPS THAT YOU MUST DO. ONCE THIS IS COMPLETE, THE SEQUENTIAL ORDER OF THE COLOR CODED WIRES WILL BE THAT, WHICH APPEARS IN THE PICTURE, ABOVE.
How does your now differ from that picture?
If you can remember where each was or took before pictures you should be able to start at square one again and try it again.
Here is the referenced wiring diagram from the article so you can follow the wire color here and find what it goes to. Its black and white but you can always print it out and use colored highlighters or steal your kids crayons and trace the color wires you need.
only 12 wires so it shouldn't take long to set it straight. Just be patient and make sure you have good light so you don't miss the stripes on the wires.
To swap the wires, you release the wire contact from the plug on the end of the wiring harness. You can release the pin by inserting a very small screwdriver or paper clip into the connector and depressing the locking tab. Once you release the tab, slip the wire contact out of the plug and bend the small locking tab back. Insert the wire contact into the designated position (listed above), it will click when it engages back into the connector).
Double check that the wires in the new positions are locked in to the harness connector. Loose connector may be the issue.
The volt meter has its' own spade plug connection. You can just run a lead into the "IGN" terminal of the fuse box which supplies power only when the key is in the "on "position.
Where did you pull power from for this?
tach goes into pin 1 (just run a wire from the distributor [TACH] plug on HEI. If you're careful you can re-use the harness pin from old Pin 4).
Then what did you do about the tach connection? Did you reuse the old number 4 wire or run a new lead to the tach through the fire wall? If you ran a new wire is the old number 4 grounding out or is it taped up neat?
You must purchase and install a new temperature sending unit (FOR GAUGES) for the temperature gauge to work properly. ( I purchased a "Standard" brand, stock number "TS6", for about $12.00, from my local parts store.
Did you install the temp sender as required? It could need to be grounded by the block.
Hope it helps you get either to the finish-line or back to square one for a second pass at it.