Phone Dials came in some different sizes for either the 944 or 928:
They were either a 15 or 16 inch wheel and either 7 or 8 inches wide.
Offsets for the early 944's phone dials were 23.3
late 944's were 52.3
928's were 65
Probably what's needed for the Boxster would be:
front - 16x7 with 53.3 offset
rear - 16x8 with 53.3 offset
Early 944 wheels (23.3) won't work. They'll sit too far outside the wheel well.
928 wheels (65)
might work, but they may sit too deep and have interference problems. Spacers may/may not fix the problem.
Late 944 wheels (52.3) should work great. If I remember from my 944 days, 16x8x52.3 phone dials are kinda tough to find.
You may have to play with 5-20 mm of spacers to make everything look right.
Also - the wheel width and offset should be stamped on the backside of the wheel on one of the spokes. They'll say something like: 7Jx15 ET52.3 or some variation of the numbers above.
Having just w/in the past year put new 16" tires on my '90 944, they are pretty hard to find with the correct size/speed rating,
I'm pretty sure the above info is correct, but if someone knows better, correct me.
Rick