I have the 75ius on that plane, and it is an amazing setup!
just to make sure I got the right engine model - I'm talking about the rear intake type.

I fits into the cowl when installed inverted, when the cylinder head and plug cap sticks out (picture enclosed).
I did have a bit of a froblem to fit the mountinf plate onto the firewall, if you don't intend to use it it's fine.
FYI, I didn't use standoffs because of the CG, meaning I had to open a pretty big hole in the firewall for the carb,
plus, i hade to sand one engine box corner to fit the pitts muffler (pictures inclosed).

for simlysity - go with the US, the side mounted carb will make it easier on you to install, but keep in mind thaat the IUS/QS are more accurate and smoot, as most rear intake are.
as much as power in concerned - the 6.5hp US will take this plane easily to 3D,
but THE IS NOT SUCH THING AS OVERPOEWR!!! if you wand to take her all the way - go for the IUS/QS.

This was my first big one too. no doubt - the best choise!!!

what is your system setup?

GOOD LUCK!!!