Greetings Dave. I also have a 1955 Mercury Mark 25 electric start and would also rather install a Delco starter as opposed to the “swing away” starter given it’s more reliable and easier to install, (plus the 1955 is a positive ground starter). The issue with the 1955 model is the Delco starter given it’s design won’t fit given the top of the 1955 model uses a standard Mark 25 recoil/top, the 1956 model had a different cast top designed for the “taller” Delco starter, also the port side cowling of the 1955 may not clear the Delco starter. Only answer I see with using the Delco starter on a 1955 is to gently remove enough material on the aforementioned side cowling and the top to make clearances for starter. Always appreciate your thoughts on these Mercs. Shell in Oregon.