Adding a temp gauge is very easy. Your thermostat housing should be tapped and probably has a plug in that tapping currently (if it is FWC it will be on the top front of the engine). If it is a RWC 5411 then this tapping is on the front of the t-stat housing as seen below. I can't recall the tapping size but that is easy to figure out.
The factory calls for a 299066 temp sender but they want $80.00 for them. Any auto parts store will have the 33-240 ohm sender you need. All you need to do is match the correct threads.
Once you have the sender in buy any Faria or Teleflex temp gauge and install it. One wire goes direct from the sender to the "S" terminal on the temp gauge and the other to system neg. There will also be an ignition terminal and back lighting terminal which are both fed by the key switch. This is a couple hour job at best and under $50.00 in parts.
This should be the temp Sender Location on RWC 5411 engines.
SBO has the Faria gauges & temp senders.
The temp is only $27.79 it is part
#S-183719
A 1/8" NPT Temp Sender is part
#S183775 and is only $10.37