I thought I would provide info on the nacelle firewall measurement if it might help.

From the firewall to the nose ring it s 4 3/16 inches.

I chose the YS 63 for mine and it is 97.5mm from back of the motor to the prop hub. It is a bit hard to see in the picture attached but there is about 1/8" or less from the back of the motor (throttle arm) to the engine mount bolts and that leaves about 1/8-3/16 sticking out from the nose ring.

A Du-Bro spinner that is set back will eliminate most of the gap in the front.