I am not sure with regard to the water pump. I used a water pump of a 235 insert engine because that is what I had laying around. It might be helpful to post a few pictures to get some better answers.

The difference between the starters lies in the voltage they use and the starter drive (bendix). If you are using the original starter with your 1955 261, it will be 12 volt and I believe the flywheel ring gear teeth are finer. Anything older than 55 second series in pickups was 6 volt. The 261 I put in my 51 had a starter meant to be operated with a push button. I wanted to keep the stomp starter, so I had to use the flyweel and 6 volt starter from an old 235 I had laying around. I am running the 6 volt starter on 12 volts and it works fine. The key thing to remember is that if the starter has a solenoid on it, it is meant to run on 12 volts and the flywheel ring gear (and starter bendix) are different than the old stomp starter setup.

Remember 9-11-01--God Bless the USA
JUSTICE, not REVENGE, will prevail

1951 Chevy 1/2-ton Pickup truck