cooling my slant 6

For diagnostics, simplest is a Harbor Freight IR gun, or a "temp gage in radiator cap" (ebay). I don't know what a "mechanical temp gage" means. Mopars had either an analog gage (resistance vs temp sensor) or dash lamps (on-off switch sensor). My 65 Newport has "cold" & "hot" lamps from a single sensor. The switch type should work for any bulbs. The analog type must be exactly correct, plus your dash "voltage limiter" affects the reading.

If the valves are adjusted too loose, you will hear excessive clacking. If too tight, you won't get full compression. Either makes the engine runs less efficient which can cause over-heating. You should adjust valves every 30K miles or so, so "just do it". But, in 1978 you could have hydraulic lifters, which have no adjustments. They started around that time.