I don't think bolting the motor to the firewall will get you past the cowl.

I wouldn't use the plastic GWS mount.

I would make a fixture consisting of a 1/8 ply firewall large enough to mount the motor to plus some extra, and another piece of 1/8 ply at a right angle on the back and in the center, with gussets to the motor mounting plate for extra strength. Make this horizontal piece the same width as the mount you already have and deep enough to cover about 3/4 of it when the vertical part is slid back against the front of the rails. This whole assembly could then be slid fore and aft on the existing mounting rails till you have the correct clearance for the cowl, and then screwed directly to the OEM mounting rails. This will also preserve the designers thrust lines for you.

In other words, take advantage of what you already have to make it easy to set up the motor. And you could always mount a larger motor to the same rig in the future.