First - there is no such thing as “explosion proof flex”. They are “explosionproof flexible couplings.“ Technically, the 36” limit is a listing restriction, meaning no NRTL will certify such a fitting longer than 36”.
If you could get one, it would be prohibitively expensive.
See Sections 501.10(A)(2) or 501.10(B)(2) for recognized Hazardous Location options.