Just so you understand what is going on. The regulator keeps fuel pressure steady when compared to manifold pressure so that high vacuum does not "suck" extra fuel from the injectors.
If our fuel pressure gauges referenced manifold vacuum instead of atmospheric pressure the they would read steady.
Since they reference atmosphere they will show 43.5ish with no vacuum or mid to upper 30s idling with the vacuum connected.
If pressure is dropping that much at high rpm I would start with a filter change.
I thought specs were 43.5 @ idle & 50 @ WFO IF it was my car it would get 2 things 1) walbro 255 with hardwire kit from racetronix 2) Adjustable fuel pressure regulator set @ 43.5 idle With nitrous Wet or Dry you can blow **** up if your fuel pump doesnt keep up Then youll have a bad taste in yer mouth & miss out on all the fun lol
Adjustable regulators for most of us are a waste of money that complicates things and makes them less reliable.
Stock regulator works just fine with a Walbro 255lph pump, I wouldn't really consider replacing it until looking at a very high volume system or boost.
I say this having had an adjustable regulator on the car for awhile.
Your loosing a bit to much pressure on the top end so If the filter does not help you can get a regulator for about 30 bucks at one of the parts stores that is a replacement part or pay a bit more for the GM part. I had one go bad when I put the stroker in and replaced it with one of the parts store bought ones and it works perfect now. If that doesn't solve it then it may be time to look at a walbro style pump to swap out the stocker. I wouldn't run it on the NOS until you fix it since you may be running the mix lean on gas.
__________________ 97 Camaro Z-28 stock 350 internals with GM white spring W/LE2 heads and cam, CAI, Borla with E-cutout, Lpp LT headers & ORY, Monster stage 2 4l60-E (POS!!! on my 3rd one), Jegs under drive damper, OBDI conversion, Madtuner, Stock 3.23's