It'll break the scale lines, but I'd say go prop-in-slot, motor in pusher format on the CG. You'll have to tack down/stramline the trailing edge of the slot to keep it from blowing out in the propwash. you'll also need to re-enforce the sides of the fuse, but shouldn't take much.
If you want to go with the typical mounting, it'll look out of scale, but build up a foam block out of 3-4 layers of stripped DTFB and glue/tape/screw your firewall to that.
If you want to go with a drive shaft from more scale lines . . . good luck. It can be done, but it'll take some machine work and it's a nasty part to bend/break . . . got any carbon fiber tubing and matching bearings/bushings for the shaft? On that airframe, it'll also be a bear for thrust angle, since you'll probably need more down than you want to keep the motor bell inside the fuse . . . or a really long shaft.
Whatever you choose, looking forward to seeing it take to the air