All you need in order to use an original style 56 truck electric temp gauge is a temp/amp gauge
mounting plate from a 56-59 truck gauge cluster. That 3 screw plate has the "hump/recess" to accept and mount the electric gauge. Probably will have to buy a whole truck gauge cluster from Ebay, Craigslist and etc. A cheap cluster that is in bad shape but has the plate....or see new plates below. Maybe someone sells new plates without a gauge.
Old style With "flat plate".
New style recessed for electric temp gauge.
Maybe Classic Truck Parts has a return or can get you just the plate. Obviously someone is making them.
I notice the 55 temp gauges have a three dot face. The 56-59 has two dots with a white line in between. Aftermarket are all three dot, but there are gauge face decals available for 56-59 die hards.