This morning, I decided to install the engine mount, it went quite well. Six 3/8" bolts will maintain the mount to the firewall. The four outermost holes are drilled in the firewall but not in the steel brackets that are inside the fuselage. As those holes might not be well aligned with the mount holes, they have to be drilled in assembly with the mount.
I first outline the mount supports around the holes (blue line in the next pictures) and then clamp the mount to the firewall through the firewall recess hole.



Here is a picture through the upper mount on the right side, the structure behind the hole is not drilled yet. The other holes are a bit less centered (less than 1/16 off) but all are OK.



Drilling to 3/8 directly is not possible, so I first drill to 11/64 using a plastic bushing found at a hardware store in the US.



Then I drill to 1/4 using the bushing I removed from the right stick yesterday. (I had some steel bushings that were supposed to be 3/8 but were a little more).



Then I install 1/4 bolts before drilling to full size. As a bushing, I use some 3/8 aluminium tubing (I have a lot of leftovers from the fuel system).



The four corners being bolted, I drill the two central holes to 3/8 using the same steps as for the other holes except the 1/4 bolt.



Then I drill the four corners to 3/8. Here is a view from inside of the fuselage with two big lower bolts..



The parts to drill being quite thick, cutting oil must be used regularly and the shavings must be cleaned regularly too.