First a congratulations, and a sincere thank you, for your success in bringing the 285/50VR15 re-issue to market.
Yes the 225/50VR15 and 285/50RV15 P7's fit all narrow body cars the same. I guess that's your main question. But there's a difference in tire diameters, and the smaller P7's don't fit the wheel wells (designed for larger tires) as closely as Tjaarda had originally intended. The gap between the tire and fender is a bit 4WD-ish. Wheel off-sets are a bit wrong too. Instead of redesigning wheels or fenders, De Tomaso just made do with existing parts. Some people like that look ... it was after all the factory look of the GTS (narrow body Panteras) of the late '70s and '80s.
Here’ tire pressure info: