Car won't run past 60mph

If too tight, it will be pulling on the kickdown lever and making the trans think the throttle is hard open all the time, and the trans will stay in a lower gear and the shifts will be at abnormally high RPM's/speeds. (But I do not think that would not explain the high revs in reverse.....??)

The correct Lokar adjustments have nothing to do with the throttle spring tension, so I am not sure what your post above means exactly. The kickdown lever on the trans has some light springing inside that returns it to the closed throttle position; that maybe the reference to light spring tension. But it would be VERY light return spring tension only; you really need to get under the car and check out the kickdown lever motion and adjustment. Moving the kickdown lever on the trans rearward raises the shift point RPM's (like with more throttle opening).

BTW, where/how is the kickdown cable connected to the carb throttle?

All these suggestions to check the kickdown are just a starting point for the easy (dumb?) things to check first; it could still be in the trans, TC, or elsewhere.