Did the SCCA rules require B-Production racers to race with a spare tire or did most r-model drivers ditch the spare tire when racing to save weight?
No - SCCA wasn't like Europe in that on track repairs were not permitted. The rules allowed for many items to be removed from the car.
From the 1965 GCRs
A usable spare wheel and tire identical in size to one of those mounted on the car shall be presented for safety inspection. Unless the Supplementary Regulations for a competition otherwise specify, this spare wheel and tire may be left in the assigned pit. Under no circumstances shall the passenger's space be used to carry the spare wheel and tire, nor must the normal working of the door be impaired.
Thanks, that's exactly what I was wondering.