Scroll down a bit.
Look for the similar label on the outside of your computer.
Quote the specific model number - that's at the end of the first line.
Quote the Product number - that's on the third line.
"..lthe now deceased 500something watt PCU into. "
How did you determine it had about 500 watt capacity ? It's the max output wattage capacity rating that's important, not the power it requires from the AC source. That's not likely to be about 500 watts for a computer with "cramped nature of the proprietary HP case " - it's probably a lot less than that.
Sometimes the power supply capacity is small enough that it can't handle the extra load a video card installed in a mboard places on it and it eventually fails.
You have several things to consider.
The first one is, when a power supply fails, it sometimes fries the motherboard,and/or anything connected to it.
E.g. Some HP systems have BESTEC power supplies - they are well known to be a lot more likely to damage your mboard and/or other components while failing
If the mboard has a standard sized main power connector, try connecting a regular sized power supply to the mboard and drives to see if the mboard still works fine - you don't need to install it in the case for testing purposes.
If the system will NOT work with another power supply, that's a whole different situation
If the system works, then you have this problem
Brand name system builders usually provide NO INFO AT ALL about what pins on wiring connection headers on the mboard are for what - e.g. the front panel header for the power switch, power led, hdd led; the USB headers.
You need that info in order to connect the mboard to a generic case.
HP did/does not make the mboard. Once we/you have the proper model number / Part number then usually the actual maker of the mboard can be determined and you can find out which pin is for what on the headers by examining the manual for a similar retail mboard model made by the same mboard maker.