Which seamaster 300? I doubt it makes sense to install such a substantial chunk of metal on a vintage seamaster 300 even if the endpieces somehow fit. You may have misplaced your post.
This Seamaster 300: Which, you can see, happily wears a 1610 bracelet, among others. This is the 98000144 rubber strap: This version is from the 42mm Planet Ocean. It is rumored that many Planet Ocean straps fit the Seamaster 300, which is "the same" case as the Speedmaster Professional, which is why the above bracelet fits the Seamaster 300 using 840 end-links. I'm in the right place. Tom