Then if no other changes, I'd look at:
a) stock bar with 'polygraphite' or greaseable polyurethane bushings and poly endlink bushings with heavy washers and tube. Thicker and stronger metal washers are needed with the polyurethane bushings. or
b) Next size up that will fit. 1 1/8" is pretty common. I think anything between 1 and 1 1/8" will work reasonably well with performance oriented all season radial.
Firmfeel's stuff will fit. Firmfeel Mopar Suspension and Steering
I don't know how well the Addco bar will fit on a '66. Addco has been around for years so someone here may know.
