OK the rear beam is mounted by a rear diff mount and the two donuts at the outer edges, these 3 points must all be the same spec, all3 poly or all 3 solid. Marco has sold solid beam mounts, JC motorsport and A1 RallySport too I believe, get these if you go solid diff mount. I would for track use Ask Mike at R+B while you are at it, Im sure he can sort you out a proper set up