What is really marked on the transistor? Many people have written that the 2N6087 is a typo and that it should read 2N6057.

2N6057 -> obsolete, replacement was 2N6059.
2N6059 -> obsolete, replacement is 2N6284.

PMD10K40 is also a high current Darlington pair transistor -- also an alternate for the 2N6057. I have no idea what an SH1275 is.
I thought System 6 machines used LM323K type regulators and the series pass transistor design (used 2N6057) didn't get used until System 7s.
If you have the series pass transistor design then it sounds more like the regulator (LM723) isn't functioning. This if often due to small caps C7 and C8 going bad. Did you replace these two?

You mentioned the transistor was getting real hot. Yes, it does get real hot normally. If abnormally hot - is the current sense resistor (R13?) also getting real hot? If so, the power supply may be overloaded and shutting down.

Before you do anything - can you post a photo of that board so we can all be sure we are looking at the same thing?

OK -- so 2N6057 ("2N6087") or any of it's successors don't reside on that board, those are used on System 7 power supplies.
The part on that board is an LM323K or equivalent.

Post office is definitely FUBAR right now. Fortunately, Most orders to EU are not taking that long but there are a few winners.
Last order to Netherlands took 3 months and 1 day to arrive - one day after insurance claim period. Not sure what is happening but the delay is occurring between the Chicago exit of country and the destination country's port of entry. Getting put on wrong plane maybe? Does not only affect EU, last shipment to Japan left country through Chicago - showed up in Cincinnati airport one month later.

Overloaded. Something you have on one of your boards appears to be drawing excessive current. Any hot IC's on your boards? Be careful! Having the "Motorola" logo branded into your finger tip hurts!