Unless this airplane had mods the useful load is #1427, that is fuel,pax and baggage. With a fuel burn north of 14gph the fuel cut into the capacity, six adults would be tight. Keeping your airspeed could become an issue, have not researched what the temps were when they took off, but with an airplane that was built in 1970, will be interestingto learn more as it comes available.
You’re right, 6 adults, luggage (?), fuel, possibly warm temps. Low-time pilot (?)... All factors reducing margins for error. What is it they say: altitude, airspeed, and brains - you need at least 2 to survive... (got my pilot friend and flight instructor a sign for his hanger that says that)