Mylo
See the file below. This comes from the Wheel & Tire Database put together by Mark - aka Xtreme70SS396 over on the TeamChevelle forum. You can download the entire database if interested. The file below lists only the '69's.

This is for Chevelles, but should be applicable for your Beaumont.

As Carl said, the most common rear tire for our cars is a 275/60R15 and the most popular wheel for that tire appears to be the 15x8 with 4.5 inch back spacing (which I believe translates to a zero offset). Almost everyone reports these work without modifications or rubbing. I say almost everyone because there are hundreds of potential modifications that could alter wheel/tire clearance - things like replaced rear quarters or a different rear end (66-67 rears are narrower than the 68-70 rear if I remember correctly).

15x 10 rear wheels with 5.5 inch BS (again zero offset) will usually work - but most people report that they've trimmed the lower shock mount and the clearance there is really tight. Taller wheels (16 and up) seem to avoid that problem.