Did you set the clearance in the rear piston correctly, I used to have a shim made up for assembling the 66 and 75, lower power could also be simply down to the age of the springs, these were quite low tension on these in the first place and would have now been in the rifle for many a year.

5ft lb isn't too bad for a 66, they were generally not much higher when new, mine ran at 5.8 after being fully refurbed and that had new springs.