I'm not sure I follow where you're going here - all that NARBA did in 41 was to move existing facilities up the dial without changing their relationships with other co-channel signals. WORL and WPEN coexisted on 920 pre-NARBA the same way they did on 950 after. Same with WNAC and other stations that went from 1230 to 1260, or WHDH/KOA on 830 to 850.
What's very true is that Boston's AMs overall got the short end of the stick compared to other very large markets of the era (Chicago, Philly). I suspect it's because some of the good regional frequencies that might have gone to Boston ended up in nearby big cities such as Hartford (1080) and Providence (630, 790, 920) that also needed good service.
And it didn't help that Boston was in the same radio zone as NYC and so of course the clear channels that were allocated to the zone mostly went to NYC. I'm sure rural areas of New England outside of metro Boston had no trouble listening to the NYC clears at night through most of that era, minimizing the need for big duplicate network affiliate signals out of Boston.