Oooh, having owned quite a few Mustangs including a '70 Boss 302 and then professionally restoring the cars for years I've paid attention to such things! You must have the Monogram kit or the re-issue by Revell which now includes a Mach 1 and Boss 302 version?
While MPC made a 1/25th 69 Mach 1 years ago it's OOP but included a not too great shaker option. Doesn't matter the parts won't fit your 1/24th. IIRC many years ago Fred Cady perhaps marketed the shaker? Also 69's had "scoops" in the quarter panels and now I can't recall if that included the Boss cars?
I'm not aware of a 69 Stang to at least get the front end grill. headlamp buckets (called fender extensions) and rear end treatment (concave tail panel with inset lights) let alone a good shaker. Unless you come up with aftermarket parts this 1/24th conversion won't be just cut and splice because you need a '69 donor which would result in that being the kit you should start with.
There, I've just told you nothing!
But I'd be interested to hear otherwise too?