Water temp and gas gauge not accurate while engine running.

This could be a few things. In fact, on my 67 it WAS a few things.

First, run the car at a good fast idle with the battery "up and normal," at an RPM to simulate low to medium cruise. Accurately measure the battery voltage right at the battery posts. You'd like to see 13.8--14.2, not below 13.5, not above 14.5.

Next, you need to determine if the ignition circuit is not suffering from voltage drop. The basic path here is..............

Battery -- fuse linke -- bulkhead connector -- ammeter circuit -- in harness splice -- ignition switch connector -- through the switch -- back OUT the switch connector on the "dark blue" "ignition run" (IGN1) circuit.

THAT branches off back through the bulkhead into the engine bay, and the other branch feeds power to the cluster and warning lights.

So if there is a bad connection in there, the switch connector, the switch itself, your ignition switched power may be cheesy to begin with.

"You are going to have to pull the cluster"

Unhook the battery ground, drop the column, and git 'er done.

Get the column "out" where you can gingerly get to it, maybe with a towel layed under it, etc

Now make sure everything is "safe" hook the battery back up, and turn the ignition switch to "RUN" Carefully measure the cluster switched power wherever it feeds the gauges.

While you are at this, you might as well check for voltage drop out through the bulkhead into the engine bay. Identify switched ignition power out in the bay. This should feed your red trigger wire of your MSD, and your regulator "I" terminal. To measure this drop more handily, stick one probe on the blue "igniton run" and the other probe on the battery POS post. You are hoping, here for a very low reading, NOT OVER .2 -- .3V (three tenths of one volt)

ALSO some gauge circuits are what is known as a 'bridge circuit' and those type guages have a ground post which MUST have a good ground, such as to the steering column support behind the cluster.