I also have a Nuance with a Himax 210 and know of at least six other Nuance/Prolog’s that have outrunners. I looked at my install this morning and I could have set the firewall back at least another 1/8 of an inch. Also, I used ¼ inch Dragon plate for the firewall which was overkill; 3/16 would have been plenty. So, with a 3/16 inch firewall, I could have come back at least another 3/16. To get the Himax in from the front you have to grind down the nose ring a bunch, but it’s still plenty stiff.
There actually is a way to put an outrunner into almost any plane without going in from the front. What you do is build a regular outrunner firewall and cut a hole in the center that allows the motor to slip in from the back. You then build a second “smaller” firewall that the motor mounts to. This firewall is bolted to the main firewall from behind with blind nuts. The way you install it, is to mount the motor and firewalls together and slip it into the fuselage from inside. Put on a spacer and the spinner backplate, then tape the whole concoction to the nose. You then epoxy the main firewall in from the back; remove the motor and epoxy from the front.
With this method you can put in pretty much any length and diameter motor.