340 4 barrel and 340 6 barrel: were they really UNDERrated?

Old articles out there claimed chrysler would periodically pull '68 - '70 340 off assembly line and put them on engine dyno as part of quality control. Its was claimed that they would usually see somewhere between 300 - 320 hp on the dyno.

Keep in mind the chrome exhaust tips had internal diameter around 1 5/8 inch or so, driver side head pipe had giant pinch to clear torsion bar, crush bent exhaust pipes etc. So once installed in car I would imagine a significant power loss compare to what was seen on engine dyno.