Yes they work. In 56 the gas guage is 12 volt. Some of the gauges I believe were the same through the 70's. Cluster is different but guage behind is same.

 

I am using he original gauges in both my 54 and my 55. They are quite easy to clean up and restore.... paint the bezel, paint the needles red/orange, etc.
Here's my 55 instrument panel. The face cleaned up real well but the face of the speedo has a little bit of rust showing.

 

Here is the instrument panel in my 54. I think it's a bit better but I scratched some of the paint off the bezel when I put it in place. I should have put tape over it, I guess.

 

 

If you touch the poles on the back of the gauges with a 9v battery, the gauges should swing over, if they don't move they're probably bad(don't think I've ever seen a bad one.) The senders will need to match the ohm range of the originals or they wont read right. As for the speedo, the drive/driven gears in the tranny can be changed to speed up/slow down the speedo to get it to read right.