Being a mustang fan and owner I love the idea. Hopefully he can go the full distance and add the Saleen parts to finish it off "for his son", heck for himself:lol
I can also tell you both those cars have the wrong lightbar. The movie barricade car had a low profile LED bar. If I'm not mistaken it is a Whelen brand lightbar.
I would kill to have a mustang and do it up like Barricade. I would think you could have the lettering of 'Police' and the 911 logo made as magnetic decals and mount the lightbar with strong magnets in place of the rubber footpads they normally have. Only true departure from the movie would be that you would have to run the power and control wire bundle up along the A pillar (which a lot of departments do- just not the decepticon police department) rather than route the cable through the roof as most departments do. Would make it such that you could conceivably drive the vehicle normally and in about a half hour or so have it converted to full police mode. No screws into sheet metal for the bar and no holes through the roof (those things never get sealed up well enough when we decommission cruisers-always ends up leaking or rusting around the hole) and no need to apply and remove one time only vinyl decals.