Hi Wynn,
I have used this approach several times, with "X" shaped plates attached to the motor and simple mild-steel threaded rods rather than standoffs. Blind nuts behind the firewall and washers/nuts in front combined with nuts on both sides of the plate attached to the motor work fine. I have a picture somewhere, but not with me now. I have done this with the motor mounted from the back and with it mounted from the front. I used 6-32 rod with big 3000 Watt motors and 4-40 with smaller stuff. The springiness in torsion never bothered a thing, and the stiffness in the "wobble" direction is good.
I'll go look for pix tonight,
Dean P.