This may sound a little off the wall, but on much smaller bores I may add, I have wrapped a boring bar or two with elastic bands. They seemingly change the frequency (chatter) of the tool and holder. I have run some pretty impressive L to D ratios this way. Also you may try and vary the RPM as the tool is cutting slightly back and forth to break the frequency that makes the tool chatter. As it does not stay at any one solitary frequency it might not start chattering or at least control it enough to meet your finish requirements.
Also you may try wrapping the part as well especially if you are not truly sure if the tool or the part or both together are the culprits......Anything you can do to dampen vibration will help...Including modeling clay but that is another story