Tom I have. I know right now where the Dart sets that has one in it. I have seen many others also that are definitely a 8-1/4 with sbp. They were all pre 72 cars I saw. From your description what your seeing is 73 and up cars.
Also Thanson is right about there being a few 8-3/4's used in 73 cars. They were extremely rare. You'll most likely never find it in the books anywhere that they used a 8-3/4 in a 73 car but Ma Mopar wasn't going to throw away new 8-3/4 rearends left over from the 72 model year so they used them up in early 73's. Again their wasn't many at all, but you can possibly see one. In the nearly 30 yrs. of being into Mopars I've seen 1 that I know for a fact was an all original 73 340 Duster and it had a 8-3/4. That's the only only one I've seen but I have read posts from reliable guys that say they've seen 1 or 2 also.