Checking the charts from manufacturer:
* B720 MTOW 104000, JT3C-7 engine, standard day, sea level => 2800m
* B720B MTOW 106200, JT3D-1 engine, standard day, sea level => 2000m
Probably looked at +15 ISA temp charts accidentially since that is about 3300m.
Checked other 707 variants and they are ok.