The 42's will be a bit too cold, unless the engine is running full throttle with a heavy load all the time. As the numbers get larger, the heat range goes up. Back when a wide variety of heat ranges was available, the general rule of thumb was 43 for mostly highway driving, 44 for town and country, and 45 for short trips and stop and go city driving. 43 and 44 plugs are getting nearly impossible to find.
Jerry
