You are not only at FD, you have a MADE Middle Str8. and you are only seriously behind if your OP has high MADE Str8 AND High FD.
Count that possibility.
Also, if you hold 99 flop 679,assume you play with proper Bank Roll Level, this is almost a unconditional no-fold at flop.
by "almost" i mean you fold if and only if your op show you 58 or 8T or made Flush if 679 are in same suit, while the chips are deep.
(for less then100BB, i would say it is 100% no-fold.)
If you just THINK they have those hand, you still have to call in my opinion.