Where did you get the VIN # from?
If from the LH apron - visible at the centre upper edge of the LH guard - either in front of or within a purpose cut-out - then the shell is a '68. [BTW there would be a matching VIN # on the RH apron too but you'd have to unbolt & pull back the edge of the guard to find it, there was no cut-out that side].
However, if you got the VIN # from a door data plate rivetted on the rear face of the LH door then I'm guessing that the doors are replacements taken from a '68 (clue - the internal RH door trim & door handle in the photo is definitely '68) while the rest of the car would be '67 going by the 1/4 panels (that's assuming the rear 1/4 panels are original, because with those are '67 'grilles' not '68, the latter design was totally different, dig up some photos to see for yourself). And '68s had the data plate up on the dash instead of the door.
Typically in the US doors got swapped on a mustang due to accident damage or rust & the original door data plate often didn't get swapped over, & then the car then ended up with mismatching VINs. Quite common by all accounts, have seen it myself several times.
Either way, that VIN # needs checking out to ensure no hassles come registration time.
If the shell does turn out to be '67, then there is no drama with keeping the '68 type doors - the only difference, should you wish to fit '67 trims on them to make the whole package more correct, is to drill new fixing holes for the '67 type door pulls as they have a different layout & length to the '68 type.
So, do your homework, educate yourself on these cars ASAP. You don't want to spend multi$$$$$ only to incur registration hassles, &/or end up with a mish-mash '67/'68 hybrid car that wouldn't appeal to many people come the day you decide to sell it.
Any questions, sing out - I have a '67 FB & know the '67/'68 series, as do several other regulars on this forum. Good luck, hope all goes well.