The 65/66 P/S box is the same as the M/S quick ratio so there should be no difference all other factors being the same.
I would not think the additional length of the pitman / idler arm would not produce significant additional steering effort, you may have some binding elsewhere in the system, gearbox input bearing preload, or stiffness in the various ball studs in the steering linkage or in the control arm ball studs??
I can only comment my 1970 "Boss 351" had the RPO competition suspension and quick ratio manual steering, with Koni shocks and 15x7" wheel and the early Radial TA G70-15 tires.
Once the car was moving above 10 mph the steering effort was not excessive, steering in a parking lot did require both hands ( +arms !).
However at higher speeds, 50~100+ the car was extremely stable, with crisp steering inputs resulting in predictable vehicle response.
My good friend had a 1970 Boss 302 with the same RPO competition suspension, but FoMoCO production shocks and P/S, the steering felt mushy, with less predictable response to steering inputs.
I did not feel nearly as confident taking that car over 100 mph unless I had a lot of runoff or wide pavement to use!
my humble opinion is you must decide the intended use of the car and then set it up to accomplish the intended performance.
If you are interested in a comfortable boulevard cruiser with good low speed efforts then P/S would be desired, if a stable track car then I would think the P/S looses it's attraction.
To adapt the later 67-70 steering system you would need to adapt the steering column also (as it uses collapsible column & gearbox with short input shaft) or look at one of the aftermarket restomod gearbox/column products.
regards
jim p