when is a good time to get aHV oil pump?

As long as your main and rod clearances are within factory specs, you'll be fine with the stock pump. I'd shoot for the loose side of the factory specs, but don't go nuts. .002 is a reasonable target, and you should get 65 psi out of the stock pump, albeit with a shimmed or high pressure spring.

A bigger pump will not give you more pressure any more than a high pressure relief spring will give more volume. High pressure will suck more power than high volume, but both will place undue loads on the engine internals. Think about the load path of the power to drive the pump... From the crank through the timing chain, end-to-end down the cam (which will twist just like your torsion bars retarding valve timing on rear cylinders slightly) through the intermediate shaft and finally to the pump. Higher drive forces mean higher friction, and these loads increase at the square of RPM. Similar effect to running too much valve spring, horsepower wasted due to internal friction.

So, how to tell if your oil pump is adequate? First Mopar small block oil systems are pretty good, but there is a simple experiment to illustrate how good one is working. Put the trans in 2nd and slowly accelerate from 1500-4500 RPM or so, watching the oil pressure gauge. The oil pressure will climb with engine speed for a while and then level off, climbing no higher. This level off is the point where your bypass opens. At this point your pump is delivering more volume than the engine can handle, and excess oil just goes back to the pan. A bigger pump would do little, other than cause the bypass to open at a lower RPM. Typically by 3000 RPM you will have 45-50 PSI, but each combo will be a little different. If the pressure levels off, but begins climbing again at a significantly higher RPM, then the pump volume is overwhelming the capacity of the bypass. If pressure levels off and then drops at high RPM, your rod bearing clearance is excessive and all the pump in the world will not save your engine from blowing up. Now, if you want more pressure (the 10 PSI per 1000 RPM is a good guideline, but don't lose sleep over being a bit under) use a stiffer bypass spring. This will allow the engine to develop more pressure as RPM rises until again the pump delivers more oil than the engine needs and the bypass opens. Pressure is a good way to visualize how well the oil system is working, but more isn't always better. Sure 60 PSI might make things last longer at 6000 RPM than 30 PSI, but this does not mean that 60 PSI will make an engine last longer at 3000 RPM than 30 PSI. Developing more pressure than you need wastes energy and causes undue wear.

As to the theory that thicker oil will boost pressure, this is misguided. Thicker oil makes more pressure because of it's resistance to flow (definition of viscosity). So the pump will overwhelm the bypass and pressure will rise. The bypass always opens at the same pressure. This is why a cold engine will show higher pressure. But the same resistance to flow means the oil may not be getting everywhere it needs to go. Remember that your pressure gauge takes it's signal pretty far upstream in the system. Thick oil is a crutch, but no substitute for proper clearances. Mopar small blocks live long lives on 10w30 at 45-60 PSI. The bottom ends of these engines didn't change much over 40+ years of production because it was a good design to begin with. Why reinvent the wheel?