I think, as suggested, u need a chain tensioner, they £15 on ebay. Then it doesn't matter having a looser chain.
Also I think the chains for the C5 are heavier duty, than the bike ones, slightly wider. Given improvment in manufacture over the 3o yrs, I think the matrials for the present bike chains probably, I hope match the stress tolerances...
Hope this helps, fun working on it!