One of these mounted on a flatbed with stabilization feet could be a really badboy. Of course the ammo humpers would hate it.
IIRC, the army did have a M-61 mounted on a track some time back. Never saw one in the flesh at Ft Irwin, but saw some photos of one doing a night shoot. Very cool.
Don't know what happened to it, think the aborted antiaircraft gun project was to be the replacement.
There were some cool youtubes of a M-61 shooting incoming mortar rounds recently, some new experimental gear.