Same storage conditions as fine wine.
When younger, yes the fortification (and the quality of the grapes) gives it a good level of resistance, but at 40+ years I'd treat it carefully. The delicacy it will now have is one of the great reasons for cellaring wine.
With regard to the vintage, 1963 is excellent, however Broadbent advises there's a degree of variation and some are now moving into decline. The tasting for the Sandeman he lists in my outdated version of his vintage wine book is not great, however variability is expected in wines this old. I think most of us are pretty forgiving to old wines, as the flavours they give are often quite special.
It should be ready for drinking whenever you want to drink it and I wouldn't hold it too much longer. What I'd suggest is moving it from a horizontal position to standing upright, maybe a week beforehand (to allow the sediment to settle). When you bring it to the table, try to do so without disturbing the sediment. Then pour it very slowly (in one continuous motion) into a decanter or jug. Either use a small torch under neck of the bottle whilst pouring (to enable you to stop pouring when you can see sediment in the neck, or just aim to get around 90-95%% of the wine poured, then stop pouring and leave the dregs in the bottle (they're great for cooking with, but if the wine is great, then you may find guests happy to filter the dregs of the wine through their teeth
I hope this helps. We had a Croft 66 this year which was excellent and I hope you have as much, if not more enjoyment out of your bottle.