Go Bigger
Mark,
the main reason to go with 15" is to maintain the period correct look. On a track, that probably isn't your primary concern.
With that, this purist would say go with 17". Your tire selection is a little better and you get away from the taller sidewalls which affect spring rate.
whether we like it or not, 15" tires are going to become harder and harder to come by.
Unless the deal on the 15's is so good you can pass it up, I'd consider the taller wheel
my 2c
Paul