If you have an NOS harness and it's been stored clean and dry it should be just fine, I've never seen a bad one yet and I've been though many of them. M&H is the best harness reproduction wire source there is but they are usually just close, I haven't seen "Perfect" yet and probably never will due to different materials, connectors, wire color, or just added or lacking connections due them typically being patterned from prints rather than modeled from an actual original example.

If the choice were between used original and new M&H I'd go with new, used 40 year old harnesses are time bombs.