If the frames and drivetrains are the same as some can attest then the only difference could be in suspension. Right??? Well the weight rating does make a difference legally.
Now if all is the same and one can mount a plow on a 450/550 with a PSD in any frame length then why not be able to mount one on the Crew Cab or extended cab 250/350? The front suspension, steering and drivetrain are not a concern IMO. I don't think the frame will bend either. Not sure why you are told not to by some dealers and others specs plow package with an extended cab PSD like the one I hung a Fisher on. I do know that they handle the plow without a problem.
I would by the PSD mount the plow and not worry about it. If something drastic was to fail (this would surprise me) buzz the plow mount off and take the truck in. A The dealer would never know. My dealer did not even care. He would do work on my truck with the plow on and Ford would warranty it. Again you need to find a truck dealer not a car dealer with a few trucks on the lot.