RE: mounting an OS 91 FS into a Super Chipmunk
I try to avoid an inverted mount if I can. I mounted it out the side in both of my Goldberg Chippys.
An advantage to a side mount is that you can leave the cowl in two pieces and screw them together instead of gluing - this way, if you ever need to get to the engine or remove it, you can unscrew half of the cowl and don't need to remove the spinner and prop