The "Check Gauges" light comes on when the oil pressure is low or when the fuel is low. I don't think it comes on for engine temp.
I think there is a separate oil pressure switch for the "Check Gauges" light, so your oil pressure gauge could be working properly but you could still be getting a light.
The most common reason (based on my anecdotal reading of the Imperial Club archives) is that the gas gauge isn't operating properly.
As for some of your other questions, Imperials had special striped tires, which could be one of your codes, electric antennas were options as well, so that's possible, even likely. Tilt/tele is also a common option, so you probably decoded that correctly as well.
There's lots of info out there on the turn signal cams. As Matt said, they're a pain. I think they're also a GM part, so they're currently reproduced, but do require some fiddling to get to work right in our cars. This info can be found at the Imperial club. There's also a Yahoo! Group for Imperials. Lots of good people there (mostly the same as the ones on the site).
There's a guy in Michigan who is restoring a '68 Lebaron. Lots of good info on his website about pulling the dash, rebuilding the engine, refinishing the brass interior trim.
Norman Witte:
http://www.wittelaw.com/personal/#Imperial