Upto 1971 all the HP ratings were gross, no muffler, no fan, no alternator, and sometimes a deep sump dyno oil pan.
After 1972, this changed to net - all the parts that are needed to run a car were required to be added. The drop in compression maybe took out 5 HP. Changing the engine for the 66 - not worth the trouble.
If you tested the 66 engine to net spec's it would be with in 10 hp of whats in the car now.
