When I spoke to the Line-X guy today he said that the charges are normally figured by the square foot coverage. Since that might be somewhat difficult to figure he said that his applicator has a "clicker" on it that acts as a meter (so to speak). His ball park estimate was less than $100 per fender - Ouch! It is expensive but I've got this stuff in the back of my F-250 SD and its great. I would expect that it would last the second lifetime of the '41.
He said the the fenders should be fully painted prior to applying the Line-X and that the insides should be etched primed.
Thanks and keep the ideas and comments coming.