I have a '03 F350 with the dreaded 6.0, had egr and injectors worked on about 3000 mi. ago then all of a sudden it was like some one threw out a ten ton anchor. I got pulled over and put it in park, truck would idle but no throttle response. when I got some one to tow it home as soon as I put it in neutral the engine died. towed it home and got it strted (kinda) ran at real low idle and rough as hell. changed oil again and all filters including fuel. NO HELP, had someone read codes and said had a bad CMP, changed it, NO HELP. Read the FICM and it was good. changed the ICP and plug, Still NO HELP. So is my HP oil pump bad or just a leak at the STC fitting? Can you just change the STC or do you have to change the whole pump?
You need a way to check your HPO pressure and you can do that at the ICP connector if you don't have a ScanGauge or something similar. It's possible that it's a HPO issue, but it would be worth it for you to find a way to monitor your HPO system while cranking.
Lacking a gauge - there is a way to use a volt meter to see what the ICP sensor is putting out.
If it turns out to be your STC fitting - you don't have to replace the entire pump, but you do have to remove it to replace the fitting.
You might just have air in the high pressure oil. It takes some cranking to get it out.
However, before you just "crank away", I would measure the ICP volts. You need 0.8 volts (sometimes a little less does it) to start. A scangaugeII is about the cheapest scan tool - or you can strip some wires at the ICP sensor and measure the voltage there.