Adding rally pac to my 71 442.
Need coil water temp wire.

 

your description is a couple of unrelated items, but:

1)temperature sending wire: already there, part of your existing harness, if you are going from lights to gauges, will need a gauge sender (there is also a change made to the plug that goes into the instrument gauge.
2) tachnometer if you did not have one before, you will need to add the feed wire from the coil (aftermarket available). You can use the existing clock power and light feeds (will be there even if the car did not come with a clock) for the new tach.

All this assumes the harnesses have not been messed with too much.

 

The wiring is located in the back of the CSM in the Accessories section.