Getting started.

Welcome to the pond.
Your starter is a newer compact type and judging by the stickers, looks like it is a reman. Changing to a compact starter is a common upgrade, faster rpm and lighter.
From what you are saying the starter is probably ok because afterall it DOES spin the engine.
Could be a bad fender mounted starter relay. But if not, you'll have to start trouble shooting back to the ignition switch.

FYI, there is a separate slant six section here with a lot of info.
Also electrical so that might be a better place to ask about your non start issues.