Having worked with a few different types of belts over the years, the first thing that stands out is that they must be running in exactly the same line, i.e., dead parallel. That means if you add 3mm of spacing to one BB, you MUST add 3 to the other, or the belt WILL walk.
This is so critical that, IIRC, at least one of the major manufacturers won't "guarantee" that your tandem will accept a Gates belt "upgrade". What they're really saying is that although your earlier model bottom brackets are certainly within tolerance for a chain drive, they are not "dead parallel".
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BICYCLE - [[I]bahy-si-kuhl] - Noun :> A medical device used to correct the common geriatric condition of OFS, (Old, Fat & Slow), in a manner that does not induce brain-decaying boredom like walking or running.
2005 Trek T2000 tandem, Giant TCR, Eddie Merckx Majestic Ti, Fuji Team, Giant Revel 29er, Windsor Clockwork (Orange) fixie, and a BikTrix Juggernaut Ultra 1000 fat tire eBike