I do not like blind nuts or T-nuts (same thing) for fastening mount to firewall. The best thing is to use a good old screw/nut/lock washer.
I've never had one come loose but have had screws come loose using blind nuts. Also, I never use self threading screws to mount engine to nylon engine mount. These things come loose all the time. Use machine screw, nylon lined nuts and lock washers.

Dave...