A sustained/repeated C (in a song with ranges like this in the other parts) is singable by altos — we get them in Plenary and in Restoration, for instance. But it's a
workout, and we'll struggle if we have to do too many songs in a row like that.
If you wanted to make it less strenuous for the altos, another thing you could do besides keying it lower is giving a G chooser under some or all of those Cs, like on the first C in Plenary.
But I don't think leaving it as-is is totally out of the question. Just know that it will be One Of Those Songs that the altos really have get geared up for and put a lot of energy into.