I did the same swap years ago, mine don't spin freely either.I don't think it's to the point of loosing ET at the track though. On my 70 cuda with Wilwoods they spin like there's no pads in the calipers. I would like to hear from the mid 70 dart guys to hear how their stock disc brake units roll or don't. That would tell us if we have a problem.
it's ok to butt heads, just don't do it with a butthead
