Yeah, that's where Google comes in.
https://www.sports-reference.com/cfb/sch...edule.html
I see BC's opponents seemed Independent, and I think there were a lot. Going to Wiki:
Quote: 1892–1972: Independent
1973–1977: Division I Independent
1978–1990: Division I-A Independent
1991–2004: Big East Conference
2005–present: Atlantic Coast Conference
So they never dealt in the AAC/Big East juggling of G5/P5 thing. So that part's easy. However, in the 70s and 80s, were they considered "power"? I see your complication. There was no "Power". Holy Cross wasn't a "non-major" D1 school though when they played them in your 1976 example apparently (but UMass was).
HOWEVER: Look at the bowl history. That tells the tale. Bowls starting Popping up for them -- and some good ones, too, starting in 1982:
https://en.wikipedia.org/wiki/List_of_Bo...bowl_games
And looking at their schedule in 1982, it was P5-worthy: Texas A&M, Clemson, Navy, Temple, West Virginia, Rutgers, Army, Penn State, UMass (D1AA), Syracuse, Holy Cross (D1AA), Auburn [BOWL]). My guess would be to count 1978+ as P5-level for BC.