I hope to use an original speedo and cluster in our '56 Panel. I'm okay with having idiot lights for the oil pressure and battery charge(or lack of charge), and hope to find a temp sender that I can use in our 500 Cadillac engine to match the original gauge. Probably adapt a '56 sender to the Caddy? I will use an original (replacement of course) sender in the gas tank, so that shouldn't be a problem.
All that to ask this, do I need a voltage regulator like the one originally mounted to the back of the cluster? I do not have one. I have the little bracket it mounted in.

 

I have just been working on gauges myself in my 56 gauge cluster. As far as using the orginal temp gauge I would look at the line of new modern technology replacement gauges that Mid-Fifty, LMC, and others sell. If you order those gauges make sure you order the correct new sending units to match the gauges. Find out what your NPT fittings size is for your senders and also buy the adapters if needed. With the temperature gauge you may also need the resistor to turn the center line of the gauge from 160 to 180 degrees. I know Mid Fifty used to sell them, they perhaps still do. If your order the gauges and they ask for the part number of the resistor ask for Diane and I am sure she could find it, if not I know I have it someplace let know and I could look.
About the behind panel voltage regulator for gauges for a 56, I have looked at a lot of these instrument clusters and never seen one. Back in the day those gauges and senders were more of a current operated device. When the truck was stock from the factory they had the turn signal flasher mounted behind the gauges however. The drumming of the flasher against the cluster back panel made the clicking louder I think.

But about the voltage regulator. You do not need one. Many times modern gauges have a solid state version of one. A zener diode and resistor that is already built into the gauge cup.

Sounds like you got a great project going! Keep us posted on how its going.