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@gbj_tester
One of the selling points of ISIS was that there was a standard BB length, with chainline handles by the crank design. There are 4 lengths, according to the purpose of the chainset:
108mm
Narrow Mountain, Double Road
113mm
Standard Mountain
118mm
Wide Mountain, Triple Road
128mm
Downhill Specific
All the spindles are symmetric, so for any given crank the chainline will move 2.5mm for each step up the spindle size ladder.
http://www.isisdrive.com/isisdrive/ISIS_Drive_Standard_Document_revD.pdf
If you use a BB designed for a 73mm shell in a 68mm shell, you'll have scope to move it +/- 2.5mm from the centre by shimming under the cups, so in theory you should be able to get your chainline dialled in precisely as long as it falls within the upper and lower limits set by the shortest and longest spindles.