Spray them with Liquid Wrench penetrant (make sure it is the penetrant, as they make several versions of the product) and let it soak for at least 24 hours. After breaking several bolts on my 1930 Buick using WD-40 I tried this. The next day the nuts spun off with ease. It is the best stuff I have ever found for freeing up metal parts. I used to hate the stuff till I tried letting it soak.