I have used the brass inserts for ST 3000's and Quadra 42's without any problem. If the firewall is made from reasonably good plywood, you won't have any problem.

After I install the threaded brass inserts, I go around them with thin CA to harden the area and fix them in place.

My problem has always been getting them in, not having them come out. Here's the way I have found the most effective.

Take a bolt that fits the insert, 6-32, for example and screw a nut on it. Then screw the insert on and jam it with the nut. Screw the insert into the firewall. Hold the bolt and lossen the nut. Now you can remove the bolt without it sticking to the brass insert.

I just mounted an OS .61SF on a Kangke SK/50 this way. The plane was made for a .46, but tends to come out tail heavy, so I opted for the .61 I had available. The included motor mount was too narrow even for grinding out so I went with the Great Planes. Rather than try to install blind nuts, I put in 6-32 brass inserts. Couldn't have been easier.

BTW, the plane balances well and has great vertical with the .61.