Regulater? $60 something, couldnt believe it myself. Sorry I cant help on testing reg. have never looked into it. Maybe you need to hook pressure gauge before the regulater ? - All Buick V6 engines were the same set up no matter what brand they were put in. I'd still change your filter because you may have picked up alot of junk from bottom of tank but your problem sounds the same as mine but Id make sure its not the reg. first - less labor. If your car is that clean dropping the tank might go OK. You'll need to remove the rear swaybar I bet. At least thats how the 86 Riviera was. Forget the MAF you have a fuel pressure problem. Good Luck !!!!!!
Edit : NAPA fuel pump - $90 something. My mechanic says hes used plenty of them and not had problems. GM dealer gave me a price of like 130 but that was 2 years ago for my 86 Riv. The $90 one I put in stopped working and I never got back at it so I dont know if its the pump or one of those Riv/Toro/Reatta electrical problems.