Bob D,
Unless you want to use the parts from a 1960 motor, here’s another source for a used complete stern bracket assembly from a 1957 Johnson 18hp that will also fit your 1955 15hp.
http://thebrazilianconnection.com/57johnson18parts.html
I’ve bought lots of used parts from this site and never got a bad piece – always quality used parts.
Bob
1937 Champion D2C Deluxe Lite Twin
1954 Johnson CD-11
1955 Johnson QD-16
1957 Evinrude Fastwin 18
1958 Johnson QD-19
1958 Johnson FD-12
1959 Johnson QD-20
“Every 20 minute job is only a broken bolt away from a 3-day project.”
"Every time you remove a broken or seized bolt an angel gets his wings."