[Arran]

 If your intent is just to fix this T.O up for resale I think that making a battery pack for it is a waste of time. I don't think it would add much, if any value, to a 600 series T.O, those seem to be among the most common tube T.O models you can find. Since these were all so called "three way" portables most people would just operate them from a line cord, if they wanted one that would be practical to run from batteries they can always pick up a later transistor model T.O. 
Regards
Arran