There is a reason you get either idiot lights or gauges. They both require a sender but there is only one source for the signal. A oil pressure gauge in 1966 was probably mechanical needing a nylon eighth inch tube running from the port on the block where your current oil pressure idiot light is currently screwed into. Even if it is an electrical gauge it still has to screw into that oil gallery in the block.
I used aftermarket gauges (Autometer Sport Comp mechanical gauges) and to get both my idiot light, my gauge and my safety cut off to kill ignition if I lost oil pressure I had to tee them all togeter.
If your head has a pipe plug in it for a temperature sender then you can have two sources to monitor coolant temperature. One for your idiot light and another sender for the gauge. If not you can find a newer thermostat housing designed to accept a heat activated vacuum valve, and put the second sender in that location.
The clock is electric and just needs a power feed. The Ammeter is actually a shunted volt meter that you put inline with power out from the alternator and ground.
The factory had a special separate harness that fed the gauges and pluged into a factory harness designed for a gauge equipped car (has a bulkhead plug that the gauge harness plugs into). You can wire your gauges separately from what Chevrolet had in mind. You just have to know what wire needs to go where.
First you need to determine if your oil pressure gauge is mechanical or electric. I mechanical you can plumb it as I did with a brass T fitting. If it is electric you will have an idiot light sender and the larger electric oil pressure sender can be put on a brass T as well. You then run a wire from a 12 Volt source (switched) to the gauge and then from the gauge to the sender. Same for the electric temperature gauge.