As long as there's no oil in coolant (and vice versa) then just replace everything lol. Full bleed, new radiator (quality one), new cts ($15), new thermostat ($10) and a thorough flush out and reverse flush of the block. Would be a good idea to do your heater tap as well while the coolants dropped
The way I see it there's no point stuffing around wasting your time when (unless you have recently replaced it) every component in the system is old and could have failed / be failing. Better off replacing it all, then you know where it's at.
Just check that your thermo fans coming too, if you start the car and put your a/c on, the fans you should come on straight away full blast.
No a/c the car will heat up to about 1/2 then the fan should kick in and cool it down to 1/4.
3/4 is way to hot IMO. After replacing cooling systems in both my commodores (VR & VS) the needles never really gone over 1/2, and usually sits on or below 1/4. (Except for one freak 47 degree day the VR got to 3/4 sitting in peak traffic lol)
Good luck!