[morzh]

Russ

So far I had few if any issues stuffing yellow 630V axials into the original tubes.

One that I had, I think, was the larger type, 0.15uF and the tube was not much longer than the cap itself. That is, to do what I typiclaly do, spiralling a paper strip, putting it on top of the yellow cap and potting with plastic, I had no space left.
What I then did was, instead of cutting a strip and making a spiral, I cut a disk size of the internal diameter of the tube, slid it down the lead and then potted. This gave me enough space and still protected the yellow cap from the hot plastic damage.

And you are right, 400V will do in most cases.