The way I did the V1, and will use the same method for the V2, was to have the motor mounting holes drilled in the firewall and the
firewall fitted as close as possible in the fuse.
I made a spinner spacer ring as you describe with the desired backplate spacing and side/down thrust.
Got all this assembled and then hung the fuse by the wing tube holes in the shop oriented as close a possible to
dead straight nose up.
Once this was done and I had the spinner with spacer centered I tack glued the firewall in from the back with Bob Violett
epoxy...can't remember what it's called off the top of my head but it's the stuff you need to use a "gun" and can attach
a long mixing nozzle to the end of.
Once this setup at least 24 hours I carefully removed the motor and then ran a finish bead of the BVM stuff around the front and back side
of the firewall to fuse seam.
Worked really good.
JLK