Steve:

You approach checks with rudimentary trig. Sin2* = 0.0349. If you multiply that by 42", you get 1.466", just under 1-1/2". Half of that is the differential on each side, so the port side (looking down) is 11.25" + .75" = 12", just as you measured. The starboard side in 11.25" - 0.75" = 10.5". That's a little more than 2*, so you might want to go 11.75" and 10.75".

If you want the prop centered on the longitudinal centerline of the plane, you need to drill the mounting holes just slightly to the port side of the firewall. You can do the math as above, just use the distance from the firewall to the back of the prop. Which probably works out to somewhere between 1/8" and 5/16" to the left of the centerline. While the math is accurate, I'd still figure out a way to clamp the whole engine/mount assembly in place, then eyeball everything to make sure it looks right.

Fred