So even with the new 130gph pump regulated to 6.5psi I set the throttle at 4k and put the missus behind the wheel to watch the fuel pressure guage and watched it drop to 4.5-5psi for the short time I watched it. I can see the value of putting the regulator on the return side, and now have to suss out what's up on the supply side.

Maybe, if it's worn out. However, despite what you'll hear here about fuel pumps, Mercury Racing thinks the stock pump is just fine for that HP level - and was willing to warranty it. For example, the 525sc used it from the factory - with the 3/8 lines.

If you are running a bypass regulator, you want it after the carb. These are very simple - just a spring and a check valve, really. The idea is that they only allow a certain amount of pressure to build before the spring allows the ball to come off its seat, so the remainder of the fuel is bypassed to the tank. So the plumbing goes like this:

supply line from tank - T for primary bowl - T for secondary bowl - bypass valve - return line to tank

That makes sense.

I just pulled off the anti-siphon valve off of the tank pick up. Are these known to be limiting or problematic?

Yes, they sure can be.