The instructions posted for the speed control system were very informative. However, as you might suspect, there are a few significant differences from these instructions and the factory installed system for 1968. A few of the differences I noticed include:
a. The 67 instruction's mounting position for the speed control head on the inner passenger fender/shock tower attaches in a different position and uses a different bracket
b. The on/off switch mounts through the instrument panel where the 68 attaches below the ignition switch on its own bracket
c. There is an added mechanical brake-cut-off switch attached to the brake pedal arm, the 68 factory system uses an electrical brake light switch "jumper" integrated into the wiring harness
There are other finer details including vacuum line hose colors and installation of a separate vacuum canister depending on a vehicle's A/C and/or tilt wheel options.
However, this is a great reference for anyone looking to repair/install a speed control system. The basics of system operation and associated parts and controls are all there.
If you don't have all the parts and you're looking to locate/purchase parts, be forewarned: this is not a project to do "on-the-cheep". Some parts show up for sale reasonably often, e.g. speed control heads, relays, and the turn signal set switch as I suspect these tended to ware out and replacements were produced. My own experience proved that some of the harder parts to locate/replace included the system on/off switch and bracket, the connecting hardware for the servo control bracket (bellows) to the existing accelerator rod, wiring harness, and the correct length and configuration of the speedometer cables. As an example, there's an on/off switch that shows up on ebay every once in a while asking over $1K...just for the switch. I guess it could be worth it if it's the only part you need to complete/repair your system.
PM me if you have specific questions or feel like commiserating over the complicated/rare nature of these systems. I surely do not have all the answers, but after installing one system, I may have a few nuggets of advice.
BTW...returning to my original question...I drilled the hole for the wiring harness using one of the dimples on the upper firewall underneath the export brace. I have a few pictures I'll post later once I get them transferred from my camera.
Lee