Dannis Adamissin posted a design for"through the firewall" rear mounts for electrics back in 2011. I found out too late that my new front mounts were just not strong enough for a Cobra 3520/14. I toyed around with various alternatiives like metal brackets till I found his post. What I am asking is do they work for a motor as large as the 3520/14? I suspect they do.
Ken
First off, nice job digging up that old thread of mine!
The stresses on a through the firewall mount are the same as on a rear mount. However with a big hole through the middle of the mounting plate, its important that the perimeter of the mount is properly anchored, especially near the mounting holes for the X-mount. Another problem with a 35xx motor is whether the x-mount reaches past the periphery of the motor far enough so that you get enough material to mount to.
If the X-Plate is too small to cover over the hole in the motor mount (very likely with a 35xx other than the Arrowind) then you might try to build-in the mount. Key to this is to have a hole in the nose big enough to install the motor, and have access to the motor mounting screw through the battery box. Build-in your X-mount with access to the motor mounting screws, or use a Hunt "Hardnose mount, or even use 1/4" plywood, with brass tubing inserts and bolt the motor in from behind.
One way to GREATLY reduce the stress on a front mount is to add a rear bearing to keep the "south" end of the motor from moving around. A relatively easy way to do that is to front mount the motor, but use the bolt-on prop adapter on the rear of the motor. Get a bearing from Boca that fits the prop adapter and bake a bearing holder that you can bolt in behind the motor after it is mounted. Last I heard Crist Rigotti was routinely using this kind of system. Find one of his build threads in the electric forum for more info.