Before you go the DOT 5 silicone route consider DOT 5.1. Although it has the same temp range as the silicone (thus the confusing DOT nomenclature) it is a further development of the DOT3 and DOT4 fluids, so it can be mixed in to a system without carefully and completely cleaning out the old system. Advantages are that it still absorbs water (so moisture won't bead and collect, causing localized rust inside the system as with silicone) but has additives to neutralize it so no rusting occurs. They even have long-life versions available that can be left in the car for 10 years. It sounds like a better product to use, especially in a classic car.