[Series900]

No, you are not way off, nor are the brackets from roll center. I never measured for perfect center, yet I have no major bind. Now that I think about it, both the front and rear mounts are in a center, think about where the bumper mounts are, then translate that to the deco strip. This is a datum line for roll. It may not be the perfect center for even weight distribution allowing one finger rotation, it still is top heavy, but not unruly. If one was to build off of the front A-arm mounts, then yes, the bracketry would have to come out, then up , to a center polar axis. And the A-arm area is a common area to be bad and needing work, which the rotiserie is to be helping , not in the way. The mounts pictured work perfectly.
Another noteworthy part of rotiseriee building is , if you do not have a lift, you need to walk the car up to hieght for complete 180 rotation. If your brackets are straight out and mount to the center pivot, you will bind/bend as you jack front or rear up. If you look carefully at the yellow arms, you will see a big bolt at either end, this bolt tightens into a a large nut welded into a tube to which the straight out arms are welded. By loosening the bolt on either end the outer brackets are allowed 20-30 degrees of angle, thus permiting jacking of either end of the car, up or down. This was built way before I had a lift, and solved the problem of getting the car up and down for complete rotation. 356 bumper mounts are very low, and the car has to go up quite a bit before the roof does not hit the ground at 180. The 911 does not need as much hieght when mounted to its bumper mounts, 42" from front bumper center to ground gives me complete rotation. the center roll tubes are a slide fit with a pinch bolt for locking and I can grab the rocker and roll the car over myself. Is it perfect center? no. does it mount easily, roll easily? yes.