The difference between the two tire sizes is that the 75 series tire will be a bit taller and slightly narrower. As long as the bolt pattern is the same between the two vehicles, the extra clearance should not be an issue. I would start by trying to switch a front wheel between the two vehicles as that is the most likely place that clearances would be an issue.
my mechanic friend in washingtion state said that i should try putting the drivers side wheel on first and testing to see if it rubs. my problem is that the crushed front of son's van is facing into my small parking area. to get a jack back there i will have to push car onto my mobile parks rd. i have someone who will take off the wheels/tires from my car and try the front wheel from it the t&c. from the looks of my wheels on the 1998 mine there is plenty of space. so perhaps i don't have the right tires on my van in the first place.
All 4 wheels are the same on your son's car, so you can take a rear wheel off his van if that is easier, and try to put that wheel on the front of your van. The Chrysler/Plymouth vans came with mutiple tire sizes from the factory, but there is usually a sticker on the driver's side door that will list the tire size that came on your vehicle.