Opens Mar. 5, New-York Historical Society, 170 Central Park West (betw. W. 76th & W. 77th Sts.), 212- 873-3400; $12. What at first blush seems like a reach doesn’t seem that unfeasible when you consider the Historical Society’s métier really is just the dead, grateful or not.This exhibit, drawn from materials housed at UC-Santa Cruz’ Grateful Dead archives, illuminates the trippy aesthetic subculture to which the band gave birth, from posters to fan art. Bottom Line: LSD isn’t included. Bring your own.