There are 3 distinct 2nd gen subframes...
70-73 (no bolt holes for 74-77 bumpers)
74-75 (will bolt on to 70-73 but does have the extra holes)
76-81 (longer will NOT bolt to '70-75 cars, and vice versa, although radiator support IS in the same place requires different tranny cross member as well)
Firebird? well bolting on is the same (yearwise) but radiator support is in a different place, and length is slightly different.