a FSM will be a valuable tool to see how the circuits are wired. [or use the site [alpar maybe ?] that hosts wiring diagrams.]
the amp gauge should be ditched, but you may be able to change that up to a volt gauge conversion in the factory amp gauge location.
the oil, water, and gas gauge need a 5v signal, so keep the can you have now that converts 12v to 5, or get a new, electronic one.
just make up a mini harness [if needed] for your original gauges, and attach that to an accessory point on your universal harness.
sounds a lot more complicated than it actually is.