dabac

If your stem is 26.0 and your bar is 25.4, a strip out of a soda can is your friend.

If your stem is 25.4 and your bar is 26.0 there's nothing reversible you can do.
But you can wrap a coarse sand paper wrapped around a broom handle, insert into stem and get turning. If you get the fit close enough this can enlarge the hole enough while still keeping nice and round within the tolerances required for a bicycle.

himespau 

Yeah, there was a 26 mm handlebar I would have preferred, but I went with the 25.4 just to be safe. Pop can is a great idea, thanks.

Since I have centerpull cantilevers I think I should just be able to cut off links equal to (or slightly longer than - as one of the ends isn't crimped and is starting to fray) what I have on there now and be good to go, right? Oh, but I'll need that knob on the one side to fit in the non-clamped side. Guess I'll have to use what I have now until I can buy a replacement.

BCRider

With new levers and the need for fitting in the inlines you're going to need all new housing for at least around the bars. But at that point you'd be crazy to not replace all of the housing and the cables for new stuff. At most you're looking at another 10 bux to do it right.

And get the cables with the smooth drawn outer finish. They produce noticably less drag in the system. New cables and housings will make any bike feel like it's a lot more expensive than it is.