I looked into this the other day. While FoMoCo uses the 5x108 bolt pattern on many of it's cars (Lincoln LS, Jag S/X type, Taurus, etc) the issue is that the centerbore is too small (see Mudduck's response). IIRC the volvo centerbore is something like 61.1mm, whereas the FoMoCo bores are around 58. If you wanted to use a FoMoCo FACTORY wheel, you'd need to machine out the bore by 3mm or so. Since Poweredbyford's wheels are aftermarket (american racing) my guess is that they are not hubcentric (ie, the wheel is resting only on the bolts). Depending on who you talk to, having non-hubcentric wheels puts a lot of strain on the bolts, potentially causing major vibration issues. I encountered a similar issue on my '91 M5. The wheels from the later 5 series (97-03) have the proper bolt pattern and offset, but the hubs are like 3mm larger. To have them fit properly, you have to source adaptor rings (Tirerack carries em) which shim the overbore down to the proper size. Naturally, this only works when the hub is too big, not too small =o)
Hope this helps!
-Lloyd