Nearly impossible, Mantua based their 2-6-6-2t on the Uintah 2-6-6-2. They scaled the boiler and details up a little and put a standard gauge chassis under it. I don't know how much they scaled up the boiler, but I suspect that even if you did put it on an HOn3 chassis it would not look right. Even if the boiler was the correct dimensions, converting the chassis to HOn3 pretty much means building a whole new one from scratch.
-Kevin
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