Joined: Jun 2012
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From: Colorado
Bikes: '91 Mtn Tek Vertical, '74 Raleigh Sports, '72 Raleigh Twenty, '84 Univega Gran Turismo, '09 Surly Karate Monkey, '92 Burley Rock-n-Roll, '86 Miyata 310, '76 Raleigh Shopper
I think what you want is to preserve the length of the longer side, which means you'd want a 118mm (closest size).
And pay extra for a UN55 cartridge, instead of the UN26. You probably won't have to change it again.
[edit[ Also, that's assuming you have a JIS spindle and cranks, and not ISO. Not sure what's available in ISO.