GM-GMC: 1996 GMC Jimmy 4x4, iridium plugs, plug gap

Question
I'm replacing the air filter and spark plugs on my '96 GMC Jimmy 4x4.  I wanted to make sure I got plugs with the correct gap.  I've heard the factory plug gap is 0.96, yet all the plugs I find at my local auto parts store are 0.40 or 0.60 gap.  Will these plugs work with this engine?  It is the 4.3L 3600 Vortec engine with 47,000 miles on the odometer.  Also, when stopped at a light in gear idling, there is a soft hiss from under the brake pedal.  After 3 or 4 seconds, the rpms drop, the oil pressure gage drops for a couple seconds and the revs and oil pressure come back to normal.  I think the bellows has finally started leaking and I'm losing vacuum, but wanted to have a second opinion before spending $400 on parts and labor.  Any and all help is greatly appreciated!!

Answer
Your diagnosis of the vacuum leak sounds very likely.  As for the plug gap, it would depend on what type of plug you are using. Platinum and Iridium plugs have different gaps than standard spark plugs.  Ask you local dealer for the plug gap for the plugs you have.
Hope this helps.