If you have a facotry tach, it's running on a separate harness from the rest of the cluster. It connects directly to the distributor, power and ground. Pretty basic to troubleshoot.
Speedometer is mechanical. Pull the cable at the t-case and with a cordless drill Christmas chuck up the square shaft to the drill. Run the drill in reverse and have someone watch the Speedo. If it moves, the drive in the t-case is the problem. If it don't turn, pull the cable at the speedo and run the drill again. If he other end works, the speedo is the issue, if not you got a bad cable. Look for the possibility of a factory cuise setup as it will intercept the Speedo cable and can be the reason it don't work.
