[hemicop]

The BO29 & LO23 cars were truly a "Jack-of-all-trades" sort of car. As others have said, many became Pro Stockers, some "match-race" cars which resulted in quite a few getting carved-up and many went on as pretty successful "class" cars, running as intended in S/S or Modified Production or even in some Gasser classes., You can find pics of cars like "Psychotic Reaction" running M/P or Lambeck's car carrying "gas" (B?) designation or more often than not A/MP. These were "natural" cars for the class as they already had the big engine/small car concept scienced out from the factory & the owner's match-raqcing experience allowed him to set-up the car above & beyond the S/S rules yet be familiar with running the car all-out & however he wanted.
As I recall the original S/S rules didn't allow for any tubing or anythingyet allowed whatever tire could fit without rear-end narrowing , so Mopar was smart enough to carve on the rear quarters of the Darts to allow a larger tire to fit without having to settle for a (relatively)small slick---the 'Cudas didn't have that issue.
As to when the narrowing of rear-ends came into play, I believe that was right at about 1973 when P/S really took off, development was coming by leaps-and-bounds, so the necessity of larger tires just evolved. Actually if you look under one of S&Ms early 'Cudas you'll see a leaf/link set-up & big tires--- kind of a cross between the still used 4-link system yet with leafsprings.