I have a hydraulic thumb on my Hitatchi excavator. The circuit for the thumb cylinder has no pressure relief, so if the thumb gets overloaded (the cylinder is forced to retract) the cylinder breaks. Is installing an external relief valve on the piston side of the cylinder the best option, or would a cross over relief be a better option? I don't have a lot of fluid power experience but both these options to me have faults: 1)If the piston side of the cylinder is simply relieved to tank, will I not get an air pocket in the rod side of the curcuit and have to stroke the cylinder to full extension upon its next use? 2)if a cross over relief is used, how do I deal with pushing a larger volume of oil from the piston side to the rod side of the ram?

There's a quick way, and then there is the right way. The quick way is to install a relief valve that will dump oil from the basecap side of your thumb cylinder back to tank. You can't use a cross over relief because the amount of oil that will be displaced from the basecap side of your cylinder will exceed the volume demanded by the rod end side, so essentially it will still hydraulicly lock.
This still leaves a problem of cavitation on the rod end that will erode you seals and promote the possibility of "dieseling effect" or detonation.
The proper way to do it would be to add the relief, but also add an ani-cavitation check valve on the rod end side of the cylinder. There should be a back pressure inducing check valve on the outlet of the machines main control valve, idealy the installed anticav check valve should tie in upstream of the back pressure inducing check to provide a positive supply of oil to the anticav check valve. If you can't tie in upstream of this valve, you can add your own back pressure inducing check valve.
What model of Hitachi do you have? Usually there is an auxilary valve section that has work port reliefs with anti cavs that is used for thumb additions, which usually has a small D03 (or NG6) valve to pilot it. I just worked on an Ex-150 today that had such an arrangement.
Hope this helps.

Doug, thanks for the reply, it makes perfect sense. The machine is an EX60-2. It has the aux. valve section you talk about but no work port reliefs/anti cavs, just plugs. I'm going to rectify that...properly.