Roy,
Are you considering replacing the OE gauges with aftermarket units, filling out the same holes in the dash, or using a seperate mounting position?
I don't have direct any 1800 experience, but have fully outfitted my 122 with auxiliary gauges, barring the speedo (hoping to just repair what exists). You can find gauges from a number of suppliers in various sizes, 2", 2 1/16", 2 5/8" and 3 3/8" for standard pressure, temp, and electric gauges. If I understand correctly, 1800's gauges are Smith's, and are chiefly electric, and are angled towards the driver. Remove one, say the clock, and measure the diameter of the hole. Does the bezel seperate from the gauge or come with it? My 2 1/16" Autometer gauges came with a rubber sleeve to fit around the gauge body and sit snugly in flat Equus aftermarket 2" metal bezels. They also came with good hardware to secure the gauge to the panel from behind, but protrude a good 3 plus inches behing the panel.
I'd reccomend mechanical pressure and temp gauges, with well-sealed fittings and sturdy hardware, in their own mounts or custom housing. It would be hard for me to replace the aesthetically pleasing OE gauges, however insufferably inaccurate they were. T-fittings can be used to keep most of your old electric gauges or dummy lights operating, and auxiliary bezels can always be removed and wiring restored. Replacing the tach and spedo with new units would be very challenging I suspect, as they stick to far out from the dash face... and upgrading to an electric spedo over the mechnical may have its own pratfalls. Also, doesn't the 1800 have a lower dash pad below the metal? This would further complicate an auxiliary installation.
As for installing the gauges themselves, in a 66 122 it was very easy. I was able to use pre-existing holes in the firewall for everything except the ammeter, and retain the funtion of the stock cluster, minus temp gauge. The threads for all the senders did not require adapters, though I did have to locate a washer with one concave face to make a snug and leak-free fit for the coolant temp. sender. For your oil temp sender, make sure the one you replace is no longer than the the one removed - it may hit the oil pump. If you want a fuel mixture or exhaust temp gauge, buy 2 senders, 1 gauge and a 2-pole switch. You'll save a hundred or so bucks... but note that if you use a lead additive, it won't last long.
Repaired, rebuilt, and NOS OEM gauges seem to be readilly available for the 1800 various places. That may be the way to go if you don't want to or haven't yet made drastic changes to the dash, and then add whatever device you feel is missing as an auxilliary. The 1800 is enviously well-outfitted for engine information... but only insofar as you know the information is accurate!
I hope this helps in some way... I can attest that I want a car that has as much information to offer the operator as a B-24 bomber - shouldn't be to much to ask right?
Good luck,
-Sean R. Custer