I'm using a 7.8 NP RS:Racing Evo2 with 4 cambers and a 43 fin.
I really have no idea what it does with a fin, cutting off 4 cm. The length will be OK, I guess.
The older IS122 can use a fin with a lot of lift and the mastfoot far behind to get a looser feeling.
Don't know how it is with the newer IS117. I'm sure that the team or other people can help you out with this.
IS87 / IS101 / RRD114 / US147