Second question first! It's difficult - the first port of call is the Ordo Cantus Missae, but since 1974 there have been updates. In this case, the antiphon Go out to the whole world seems to have been given later. This thread refers. However the ever trustworthy Google throws up a setting (by Fr Webber) linked in the middle of this page. I would think that for any communion antiphon Psalm 33(32V) would match almost always, in the absence of other guidance.
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