Jeff,

I'll assume the steel Porsche wheels are 15", will fit the Syncro, and
that a plethora is 5 or more.   If so, or maybe if it is only 4, one
might ask why buy the alloys to begin with, particularly since you have
the equipment to mount the tires on wheels.  Why not try those tires on
the steel wheels to see if the fit issues are present, and, if so but
minimal, worth the trouble.

If you want to go with 4 alloys and one Porsche wheel, then there is
this question.  Are the Porsche steel wheels the same width and offset?
The wheel's width impacts the actual circumference of the mounted tire -
same tire on different width wheel will have a different circumference.
So, the general recommendation is to have 5 matching wheels and tires.

A patch kit will work and may be enough if you aren't ever out alone in
areas where you are likely to cut the sidewall of a tire.

It was my carelessness, but the only time I have ever cut a sidewall
through was in the city when I took a corner too tight and scraped the
edge of a mid corner placed drain gutter.  This was my original 84
Vanagon.  The only sidewall cut I have gotten on  my Syncro's BFG  AT
KO's didn't go through and I luckily discovered it while putting highway
pressure air in the tires in preparation for the 2.5 hour drive back to
Baltimore after Syncro de Mayo Este '06.

Just a few more thoughts.

Sam

--
Sam Walters

Baltimore, MD