Adjusting fuel pressure on new fuel system

[82 GT]

I got it started up tonight! There were no leaks at all! That holley pump is noisy as hell! I hear it over the exhaust inside the car!
We're talkin 2.5 exhaust with single chamber flowmasters!
I set pressure at 6psi, let it warm up a little and then noticed it dropped to 4psi. I turned it back up to 6 again and it dropped again!
Did the same thing all over again until it stayed at 6psi.
I went to turn the idle speed down just a tad and it make a noise almost like a detonation ping. When it turned it back up, it went away. What was that pinging sound? Is it going to want a certain idle speed now? I didn't notice what the rpms were at though.

One thing that I will HAVE to figure out later on is why my fuel gauge doesn't work anymore. I made sure I plugged the original sending unit back into the tank when I put the tank up. I checked the fuses and they're all good. I don't know but I need a gauge!
A test drive is going to have to wait until tomorrow or the weekend.