The chemical tests sound like a good idea. I don't have the equipment... but personally would go ahead and change it with 100k and that many years.

It will cost a few bucks but corrosion damage caused by worn out anti freeze would cost a lot more.

I use Prestone "extended life" on all my classics and DD's. It mixes fine with the red Chrysler stuff. At the same time, suggest you change the T stat and radiator hoses as preventative maintenance.