Dan,
What these guys aren't making clear to you is that the Yellow F-18 ductwork is sized for a Dynamax fan. The BVM fan is a smaller diameter and will not fit without major modification.
A Dynamax is about 5 1/4" diameter, and a BVM fan is about 4 7/8". I don't recommend trying to cobble up the ductwork to make the fan fit. If not done exactly right you won't get adequate thrust.

So if you want the F-18 you should get a Dynamax and OS91 for it. If you want to use the BVM set-up then you should buy a BVM plane.

Joe