They will fit with spacers, but the cost of the spacers will be pretty high.

Also I don't think 225/50/15 is the right size, the rolling radius would be
too small.

225/50/16 (on a 16" rim) would be right, or 215/60/15 on a 15" rim etc.
Teledials were available in both diameters, 15" late offset teledials are
common & cheap as chips.

Have a look here to work out what's what:
http://www.connact.com/~kgross/FAQ/944faq13.html

Your '83 944 will have an offset of 23.3mm, I guess the '88 wheels will have
an offset of 52.3mm so you would need 29mm (approx 1.25") spacers all round
to get the later wheels' centreline in the right place.

You need to confirm your current offset (almost certainly 23.3mm) and find
out the offset of the later wheels as well as checking the rim diameter &
tyre size. Take one offset away from the other to find out what size spacer
you need.

hth, Malc.

Malc.