I'm sure everyone here knows that many '60s/early '70 muscle car engines were (on purpose) underrated. For instance, the "275-hp" 340 Mopar engine really makes more like 320 hp.
Here is a very good article in a muscle car magazine by Roger Huntington.
Page 4 has the chart of the actual horsepower, but I'll list it here, too (Mopars only):
Engine............ Rated HP.......... Rated Torque....... Actual HP
340-4 bbl..........275@5000 rpm. 340@3200 rpm... 320@5600
440-4 bbl..........375@4600....... 480@3200......... 410@5400
440-6 bbl..........390@4700....... 490@3200......... 430@5600
426-Hemi..........425@5000....... 490@4000......... 470@6000
For some reason, they did not list the 340-6 bbl; I would guess it really made 335-340 hp.
Here is page one of 6: