I don't think you'd have any major worry about the frame, wedging a 135mm hub in, but I wonder if there are some subtleties you might not encounter wedging a 130mm into an 126mm 80's Cannondale.

First, is the disk the same distance in from the dropout on both hubs? That is, is the disk going to line up with the caliper? I think it is. You can answer this one just by trying it.

Second, the chainline of the 135 hub is 2.5mm further out. They're doing it on the 11 speed double disk bikes but those use narrower chains on rings that are further apart. Without that the chain might grab on the next bigger ring when in the middle/small combination. But it probably won't, because it's a triple, so the next ring is probably not enough bigger to get in the way. You might also have to adjust your front shifter indexing and FD upper limit due to the wider angle caused by the wider rear.