As I recall I took top and bottom side to side measurments and found the center to the top and bottom (horizontal). This will establish your centerline on the ply mount by measuring and establishing half way from side to side. I then measured down to find the center of the cowling prop opening. It is important that the engine be centered on the cowel, forget the firewall opening as your up and down reference, it will be close enough. Be sure to use a very sharp pencil so your lines are percise.
I used a Dave Brown mount and the markers on the mount help greatly. Mount the engine, locate it on the firewall with small wood screws and fit the cowel when you feel you have it right. You should be very close. If all fits well drill the firewall for blind nuts.
It's been over a year for me so difficult to remember it all. I did make a servo tray across the fush. at the rear just in front of the rear wing screws. it vastly shortens the control rods and strenghtens the fuse greatly. place it from side to side using resin and glass to attach. epoxy will not work well on the resin/glass fush. use lite ply placed at the same elevationas the origional rod location.
The plane flys well with a 120 AX.