The problem is that most Mondials from 86 - 88 were fitted with "metric" rims which are specifically designed for Michelin TRX tyres. These tyres use a different rim mounting whereby the tyre bead contacts the rim on an angle rather than horizontal.
Put another way, you know how a normal rim has "flat" or horizontal surfaces just inside the flanges each side for the tyre to mount on? This means that the tyre sidewall has to go into double-curvature or an "S-shape" as you move from the tread to the rim (ie it starts by bulging OUT and then needs to bulge back IN to sit on the rim).
Anyway, this makes the sidewall stiffer (=bad), and during the early 80's (?) Michelin developed the TRX system which angles the tyre mounting surfaces of the rim outwards, so that the tyre could just have a single curve or bulge & hence work better. Problem was it needed matching tyres and rims. No regular tyres will fit, and since no-one else joined Michelin with this technology (maybe they charged too much ??), it has become a dead-end. Either have to stick with 80's tech TRX tyres or ditch the rims altogether.
Some 328s would also have this problem, as metric rims are also listed for some of those models. My query is whether the concave rim will fit in place of a convex one assuming that NO ABS calipers are fitted (because all pre-ABS 3.2 Mondials use the convex rim). I know regular 16" convex rims from a 328 would fit, but would the earlier 16" concave ones?