Why would a 1955 10hp Johnson keep blowing coils?

If you are replacing with new old stock it's possible they are suffering from internal corrosion if the storage was less than optimum, or that other elements in the ignition system need to be renewed at the same time to avoid latent/hidden faults. The ignition systems on these are pretty good when correctly gapped and setup - but this is the fun of running any vintage machinery, lots of strange little tweaks required to keep them going. Treat the engine as a venerable classic and use for fun, not serious. Any modern model will be so much more economical and reliable it isn't funny.