Coolant Help
The best coolant for this old iron is the standard "green" ethylene glycol stuff in a 50/50 mix. You'll get good metal protection, raised boiling point, and of course freeze protection.
Now what do you mean by the gauge creeping closer to where you don't like it? Unless you can verify with an actual temperature, like with an infrared hand held, it's an empty statement.
Radiators work on about a hundred degree differential between coolant temps and the outside air. When it gets hot near the 100's outside, sure the temp is gonna climb, but that doesn't mean it's overheating.
Most temp gauges on the dash have a bar between "C" and "H" or cold and hot. That bar in most cases is an acceptable range to run in.