[Electronic M]

A fuse limits current not voltage...The voltage spec is almost irrelevant (a 5A 240V fuse will change with a 5A 120V and vice versa and behavior should be almost the same).

Riders indicates it is being fed through the focus coil by the 365V main B+ rail so 350V seems to be in the ballpark of what it should be.

Reduced horizontal over scan (or under scan) and blooming can be a symptom of a weak horizontal output.

The sync issues could also be caused by it (though there are many other thing that can cause sync issues). Change that tube, then address what, if anything, remains.