sidewaysstarion is correct. Direct to sensors.
Voltmeter: obvious
Fuel gauge: exact specs depend on year, but keep it with the same year range sender and you'll be ok. IIRC, RENIX 87-90, OBDI 91-95, 96 stands by itself, then I am fairly certain 97-01 are all interchangeable as well.
Oil pressure: straight to the sender (1/8 NPT thread), which is grounded through the block.
Water temp: straight to the sender (1/8 NPT thread, driver rear of head) which is grounded through the block.
Tach: 33.3Hz per 1000rpm indicated on 91-96, I am unsure what it is on RENIX era but fairly certain that it is fed directly off the ignition coil drive signal or a specific output pin on the ICU. I seem to recall the RENIX ICU tach output being a yellow wire, but you should verify that with an FSM or check your harness. Also, I should note that early Dodge instrument clusters (94 dodge 2500 pickup w/ the 5.9 for sure, likely others) use the same signal, so you could swap a dodge V8 into an early XJ and have a functioning, accurate tach simply by wiring the tach output on the dodge ECU to the tach input on the XJ cluster.
I'm unsure what voltage range the tach signal is, but it's probably either a 0/5 volt pulse train or a 0/12 volt pulse train. Putting an oscilloscope on it should clear that up in short order.
Some math:
33.3Hz per 1000rpm... how many pulses per revolution is this?
1000 revs per minute = 33.3... pulses per second * 60 seconds per minute
1000 revs per minute = 2000 pulses per minute
So effectively, the tach signal is two pulses per revolution of the crankshaft.
You could run a MegaSquirt no problem if you wanted to - I would strongly suggest only doing this if you have a 95 and earlier vehicle, or on vehicles that aren't required to plug into a computer for a yearly inspection, as I don't believe MegaSquirt speaks CCD or OBD-II and thus it'll fail you if you put it in a 96 and then have to plug into the computer to get your inspection sticker.
