Bushy,
All PA-32 Cherokee Six aircraft have a ZFW and always have! If you have any doubt read the TCDS 'Type Certifcate Data Sheet' as published by the FAA.
Read AD PA-32/42 re Wing Spar issues. The figure is based on predictive anyalsis of fatigue testing. The spar has been tested and found wanting.
When carrying out a 'predictive analysis', no account is taken into consideration for poor maintenance, heavy or abnormal landings, over gross ops, abuse of ZFW limits, corrossion, manufacturing defects. If you want 70,000 hrs on the spar then leave the aircraft in the hangar and never fly it.
The problem is that early aircraft in this series were very hard to inspect and more often than not problems with the spar were detected by simply placing a small 'straight edge' against the main spar in the vicinity of the lightning holes. In hundreds of cases this has shown a bent or deformed spar, not recogniseable externally.
The safety of these aircraft is dependent upon correct operating techniques and maintenance. CASA have a lot to answer for when they scrapped the 3 yearly major inspection. Fortunately there are operators who 'slip' the cables every three years at most and also remove the wing tanks and tip tanks (if fitted) every few years. But then again there are those that don't.