The absolute minimum is 45psi under any condition or your injectors will need to be replaced since anything lower will harm them.
How exactly did you check that?
What REAL gauges do you have on the truck?
Next, you are starting to describe more accurately what the issue is. Somewhere in your high pressure oil system, you have a leak. It could be the dummy plugs/standpipes (replace all as a set). Those are replaced by removing the valve covers. There is also the STC fitting (snap to connect) that is well known for failure. It is much the same style fitting as you see on air lines. The internals wear out and eventually the fitting gives out and you lose all pressure from the HPOP. It can fail at a slow rate, resulting in some weird issues with restarting and starting. All three of those pieces can be replaced for under $200 in parts, if you are going to do the repair yourself. If you are going to pay someone to do these, it will run right around the 12 to 14 hour range (I am guessing).
The reason why the injectors were louder is that the oil is cold and could also be under a lower than desired pressure on the oil side.
Try this little experiment. Park the truck on a steep hill, nose up after reaching normal operating temps. Let it sit for about 1/2 to a full hour. See if it will restart.
Then, try parking on the same hill, nose down and re-doing the test.
Post back the results.
Once you have those heads off, have a qualified machine shop check them for flat/straight and magnaflux them for cracking. They can mill off up to 0.008 to achieve flat. If they must remove more, you will need to buy a new head.
DO NOT use the "black onyx" head gaskets. Cool name but their repeat failure rate is through the roof as compared to stock gaskets. The gaskets were NEVER the issue on the engine. The reason the gaskets blow is due to the style of head bolts used. High cylinder pressures can stretch the TTY style head bolts, resulting in the head gaskets failing. You are getting rid of the weakness by replacing the head bolts with ARP's, don't introduce a new weakness by using the wrong gasket set.