I exchanged two rear side window interior panels from a 260 into my 71 240 they fit perfectly. What I did notice was that the 260 interior panels were slightly thicker than the240, and were less suseptable to cracking. I know this does not answere your question but I must caution you that the interior parts (at least in the back on a 240 are thin and could crack on you if you decide to try and exchange them to the 260. Good Luck
Your 240 interior will fit into your 260 with one exception - the door panels. The cut-outs for the door handles are differently located - so they will not work. The remainder of the interior will fit. I had a 260 interior that I put into my 240 - so they should be interchangeable. But, for the most part - the 240 interiors are not quite as good and durable as the 260. I believe most of the panels were thinner. It may be difficult to change them out without cracking them.