
Rick,
Those allen bolts are fake, they are molded and part of the cover... There are phillips screws that hold the speedo/tach bezel on. I've had good luck getting the cover off without having to drop the steering wheel. Some folks prefer to drop the steering wheel some don't.
The speedometer cable comes through the firewall through rubber plug and attach to the speedo head with a clip like action, it doesn't screw on it has a wedge type attachment.
If you are replacing the cable, be it from the cruise box or from the transmission, Tie a very strong String or small rope to the end of the cable under the dash, tape it to the old cable and as you pull the old cable out through the firewall the string/rope will come along with it. Attach the New Cable to the rope and pull it back through the firewall. You will probably have to remove the rubber plug before pulling the old cable out then I cut my old one from the outer ring down to the center hole. Then when the new cable was in place I greased up the rubber gromet and put it back in place.
For an old fart like me, it is easier for me to remove the driver seat, which gives a whole lot more room to work under the dash. The speedo/tach doesn't have to come all the way out to replace the speedo cable ...
I'll look later and see if I can find a blowup in my assembly manual..
Bud