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...in case you have not yet considered this, the bicycle rims you buy today from companies like Sun (the CR18's) and many others are far superior in construction design and manufacturing than a set from the 80's . They are lighter, more resistant to deformation, and are very often easier to build than many 70's-80's rims. In general, given your description of the history and current status of your wheels, rebuilding appears to be the way to go. The rim diameter you pick is kind of up to you, but 700c gives you a lot more tyre choices.