I'd say with the arrangement as far as I can see in the photos, the prop shaft is doing a fair bit of the work of keeping the engine in position, especially if any or all of the engine mounts goes soft or sags.
Also if the engine was not carefully lined up accurately in the first place, any stress applied to get that coupling engaged and fitted in the first place will have remained on the coupling for the whole of its life.
With an engine on rubber mounts I like to see a pair of UJs to take up engine movement, and a sliding spline with a thrust bearing such as a Python Drive or similar.
http://www.pythondrive.com/