Congratulations and I fully understand wanting to keep the drive train intact. Not a whole lot you can do to the 260 that will result in performance. The later model 260 heads were nice becuase you have small chambers, but the size of the intake valves are much smaller than the 289. So if you put a 4-barrel carb on it. you should stay around the 400-480 CFM area as close as possible.
A cam will not benefit you much becuase the 260 heads have smaller intake ports as well so it does not necessarily breath as well as a 289/302 would. Dont forget, your bore size is smaller. depending on the block, its a 3.5" to 3.8" bore.
Personally from a PERFORMANCE point of view keeping things intact, i would try a 4-barrel intake with a small CFM 4-barrel carb, add headers, and electronic points. I would also port the heads as this does improve breathing and performance. Other than that, you would be spending money on "flashy" things. If you do change to a different motor (289, 302, 351W), just remember that you most likely have to change the bellhousing as I believe the block will be a 5-bolt and not a 6-bolt.
Good luck ! | 
