I bolted my motor to the firewall before gluing the firewall to the fuselage. To get proper spacing and proper position, I tacked (with thin CA) four 1/2" x 3" x 2mm cf strips to the back of the spinner back plate. I then position the firewall in the fuse, put the spinner back plate on, a cutoff prop (for spacing), then the prop nut, then spinner dome. I then scuff the fuse walls with 220 grit sandpaper and clean the area with denatured alcohol. I then use painter's tape or small bungee cords to secure the spinner back plate against the front of the fuse. I then tack 4 spots of 5 min epoxy on the fuse-firewall. After that sets up, double check for perfect alignment and spacing. If all is well, I gun a generous fillet of Hysol on both sides of the firewall. Wait 24 hours and that chore is behind you.