Wait a minute. Are we talking about a 68 or a 69? (You have both in your original thread, but since you refer to it as a "Tic-Toc-Tach", I was assuming you meant a 68 since this wasn't available on a 69.)
If it IS
a 68, then it sounds like he installed a tach and didn't bother with the other gauges (or the console). Since repops are now available, things like this are going to start popping up more and more.
If he installed the tach in place of the gas gauge, then he's not only lost that ability, but the ability to monitor the alternator and the cooling temperature as well (see below). Of course, aftermarket gauges have been added as you stated, but he's still lost his fuel level and charging system monitors.
Interesting that the adapter was still available from GM. I knew the speedo gears were, but I didn't think the adapter would be. you just never know...