Tom;
I rebuilt the rear leaf springs in my 56 Customline which I think are pretty similar to your 54 Merc. You can also have the leaf springs re-arched (which I did on my 56 F-100) but I didn't have that done on the Customline. In my opinion, I don't think that you need to lube between the individual leaves of the spring set. Also, regarding powder coating, remember that the steel needs to be heated to over 400 degrees in an oven for the powder coating process. I'm not sure if that would "un-temper" the steel, but I wouldn't want to chance it. Eastwood Company sells a ruberized undercoating that will flex without cracking that I have used in the past with good success.
Rono
Ron Lane, Meridian, ID