No oil pressure on startup/headerbolt question

For the oil pressure, you going by the stock gauge?

As far as the header leak, are you sure the leak is coming from the gaskets?
Headers can leak at the ball flange.
I've even had crappy mac headers that leaked in the collector, only way to see it was to remove the header and hold it up to light.

Did the guy that did the stud change the gasket for you?
Any time a header leaks the gasket has to be changed because the exhaust burns through it and it's permanently damaged.

Just some initial thoughts on this oil pump issue. To me, it sounds like your pickup tube is loosing its prime and is having to completely refill itself with oil before it gets the pressure up. If your looking at a lazy electric oil pressure needle it could be a instrument issue. The most effective way to diagnose a real oil problem is to install a good known manual oil pressure guage. This may solve what looks to be an issue.

Next thing is that the cylinder walls don't get oil directly from the pump. They get oiled by what is slinging around in the pan (windage) and from the .010 in ish clearance in between the pressurized rod bearings. Its hard to say if what you hear is a function of the slow oil pressure. I'm thinking that the noise you hear would still be there even if the oil pressure popped up immediatly to full blast but may be there as a result now of the a possible oiling malfunction.

For now, I would install a mechanical oil gauge and see if its still 2-3 sec slow, cold, and if so I would say your pick up tube is either loose, bad gasket at pump, or the pump doesn't pump what it should and is tainted somehow which allows it to do a subpar job all the time but is mostly noticable at the beginning. Something might be allowing the pickup tube to be dumped. As far as oil level goes, if you have an after market chrome dipstick in there, that may be off by alot or alittle, drain the oil out of both sumps and fill it with 5 quarts. That takes oil level out of the equation. If you are over serviced your oil pressure wouldn't be slow though, and low oil wouldnt yeild a slow to pressurize condition either so... oil level in this case probably isn't suspect.

I just rebuilt an engine that when cold, had 25 psi oil pressure, and when it got warm it went all the way to zero. This motor had massive crank trauma the all 13 bearings were copper everywhere, the crank was unturnable due to deep ridges on all journals especially the first, the rear thrust face was molested so much it is hard to explain with typing, and even this abused monster still had instant 25 psi on start up every single time, when cold.