Folks,
At the risk of a bit of controversy, and having flown B707-300 and -320 modified as required to be UK registered, (total about 8000 h on various versions, on various registers) versus the FAA certified aeroplane, it is my opinion that the ARB required mods. were overall detrimental to the operation of the aircraft.
I think we all acknowledge the B707 (and DC-8) had shortcomings, and aircraft design advanced by leaps and bounds over the period in question, '50s through the -80s, but the demanded changes for the B707 were, as far as I am concerned, largely a safety minus.
We should also acknowledge that aircraft certification standards have made great strides since the original SFAR 422b.
Tootle pip!!