066/660 both take the same "medium" Stihl mount (3003). The "large" mount (3002) is for saws like 076, 084, 088/880, but can be made to work on a 660 with an adapter and possibly a new oil hole drilled into the bar. In either case, changing the sprocket tip from 404 to 3/8 is cake, especially on the oregon bars as there is only 1 rivet. 5 minute job for a drill and a ball peen hammer.
I personally think you should keep the bar and change the tip. Good luck with whatever you decide.