My dash cluster (With Tach) and round vents ,had the water temp gauge crap out, which also crapped out the fuel gauge too!!: as both share a "Common choke" ( Voltage reducer"). (87 Sammy)
Now my solution was to remove the entire dash, it is not as hard as you may think!!!
I massaged the 3 center openings (2 vents 1 clock) to accept a 3 round gauge set. water temp, oil pressure, volts.
Moved the clock to the right outboard vent opening (Its the spotters job to keep track of the route time anyway)
and installed a round "Steward Warner" fuel gauge in the left outboard vent opening. This is the only real work involved as you will have to drop the fuel tank to install the matching sending unit as the stock Sammy sending unit will not work with the Steward Warner gauge. (good time to clean out that 20 plus year old tank anyway)
Original venting was directed towards the floor area by "Bobbing" the original vent hoses as I found that you were not getting much "Fresh Air" anyway!!
If you have a "Square Vent Dash" you can go the cluster swap route with basic "Idiot oil pres., charging lights" or go on a Junk Yard Safari with a couple of your buddies (which can turn into one of those campfire stories for eternity) and "Cannibalize" a round dash unit.
CAPT