Folks,
I've flown every variant of the 707, except the RR Conway powered -300, and the QF 707-138 (not a -120, as many seem to think) and every one featured Mach trim.
The biggest variable I found between them all was the yaw stability, ie; the propensity to dutch roll, followed by the pitch/speed stability on approach, the -320s could be very touchy.
Some strange modifications insisted on by the UK ARB didn't do anything to help the G- registered, although the late D.P Davies would disagree --- but he didn't have to fly them in the real world.
Tootle pip!!