Change from an alternator/battery light to system voltage indicator requires some wiring changes:
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Low oil pressure warning light to oil pressure gauge gets a little involved. In a non-gauge equipped truck there is one oil pressure sender (behind the distributor). It has two functions;
1. Illuminates the dash warning light when engine lube oil pressure is low. It alarms upon falling at (about) 10 psi.
2. Provides power to the electric choke heater as soon as oil pressure is established. Closes upon rising at (about) 10 psi.
Trucks with gauges require 2 oil presssure senders - a on/off type that closes at 10 psi and a variable 0-60 type. Both are mounted on the brass 90 behind the distributor.
The first one is the same sender as used in non-gauge applications. Like the non-gauge trucks, it powers the electric choke heater and illuminates a dash warning. But in low oil pressure situations the lamp that lights up on the instrument panel says "CHOKE" not "OIL PRESSURE".
The 0-60 sender provides a variable resistance to ground and is used by the oil pressure gauge.
A couple of images:
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The bell shaped sender is for the variable one for the gauge and has only one lead. The one on the drivers side is the on/off switch type that supplies the choke heater and the CHOKE warning light (or in your current situation the LOW OIL PRESSSURE lamp).