The cab is the same as a pickup. Chevrolet merely bolted bigger fenders and bigger hood to a pickup cab to make it into a bigger truck. Most fire trucks had Mico brakes for parking. They are still available. It looks like you have a Spicer 5 speed. You would probably have to find a parking brake for a Spicer in a salvage yard. It appears that you have a Spicer with a 5th direct. The ove rspeed light is to keep from over speeding the pump while pumping water. It is not needed for what you are doing. GMC came with an optional SW tach. I think the fire truck body upfitter probably installed the tach because pump speed is critical. Chevrolet trucks usually had an aftermarket tach on top of the dash when they were used as a truck tractor. I have a 59 Spartan 100 truck tractor that I have never restored. I will try to think to look and see where the tach is mounted. I bought it at auction about 10 years and all I have done with it is haul it home to the rest of my collection

https://photos.app.goo.gl/hDys34fgyDoEFnh97