Much as I like the Triple-7, I have no doubt whatsoever that it would not be a part of Pan Am's fleet. Here's why:
In the mid-80s, Pan Am developed a strong relationship with Airbus, getting discounts on A300s, A310s, and A320s (which were later cancelled and delivered to Braniff II). We won't go into the various reasons why they'd gone for Airbus instead of Boeing and McDD.
If PA had continued, they probably would have flown the A300s and A310s through the 90s, continuing with the 727s until the late 90s, and perhaps even til today. I honestly believe that they would then have gone with the A320 family for short runs. The 747s would probably have been retired at the earliest possible convenience (ie, mid-90s) and replaced with 767-300ERs, given the nature of their remaining routes in Dec. 1991 (MIA/JFK-South America, and MIA-CDG). They might have picked up some secondhand 737-200s or 737-300s in the early 90s to serve the short-run routes to the Caribbean and intra-florida/southeast. These aircraft probably would either still be flying today or have just been replaced within the last two years with A319s.
In short, if the original Pan Am were flying today, I think their fleet would consist of: 767-300ER, A300, 727-200, A321, A320, A319/732/733. The Express division would probably still have been running out of MIA, with ATR-42s and J-31s still plying the Bahama routes.
I'd like to say that PA would still have been running large aircraft like the 777 or the A340, but practically, it would have been impossible. PA would be, essentially, what TWA was a year ago--an airline flying a small, inercontinental fleet, trying to stay solvent and recreate itself in the face of staggering competition.
Of course, it's all a moot point, anyway, because there really isn't any way that PA could have survived to this day, short of a divinely-ordained miracle.