A '66 Biscayne was a natural 'A' car until the '68 season when it went to 'B', but dropped further to 'C' and then 'D' stock before finally resting at 'B' in '73. Anyway, here's what NHRA showed back in the day, and that natural class(es) each year for a '66 L72 Biscayne...
Original NHRA factor page(s) - 8.56 [stock] or 8.66 [super/stock]
1965 class breaks - AA/S [note: assuming an early '66 built car during remainder of '65 season, if not in class guide they would weigh the car @ track & divide by advertised HP to determine class]
1966 class breaks - A/S
1967 class breaks - A/S [would've added weight to reach 8.70/hp factor] or SS/C
1968 class breaks - B/S or SS/D
1969 class breaks - C/S or SS/G
1970 class breaks - D/S or SS/F
1971 class breaks - D/S or SS/F
1972 class breaks - A/S [would've added weight to 9.0/hp?] or SS/F
1973 class breaks - B/S or SS/F