There are no decent independent shops in the immediate Princeton area. Princeton Porsche has a couple of air cooled guys on staff that could probably help you.
However, go to Phoenixville PA (about an hour from Princeton) and have Bill Boys at Possum Hollow Motorsports do the PPI. He has been around these cars for a long time and knows air-cooled 911's like the back of his hand. He's an honest straight-shooter as well. I take my 993 there and wouldn't have it any other way.
http://www.possumhollowmotorsports.com/
(no affiliation other than a happy customer)...good luck!