I know this reply is almost a year late, but hope it will help someone else with door panel replacement. Recovering the door panels is not an easy, quick job, especially if the boards are changed. As discussed above, the boards plus thick material make it impossible to get the panel into the chrome channels. Add to that changing the aluminum top that is riveted to the board and you have a recipe for frustration. Some handy hints. Top alum piece that rivets to the board also holds an anodized aluminum finisher with a notch in its leading edge. This notch has to match up with the chrome piece at the top front of the door where the glass slides up/down. The alum part riveted to the board has to be in the exact right spot so the board will fit into the channel when covered with material. If the covered door panel does not fit all the way down into the channel and the top of the door panel is sticking about the felt window cushions, the chrome finisher on the end of the door will not go into place. If it does not go into place it will hit the chrome cap on the end of the convertible top boot as you close the door.
My panels were professionally recovered with the car in their shop, and it came back to me with ill fitting door panels. To fix this problem I dry fitted the finished door panels by hanging them at the top but allowing the bottoms to not go in the channels. Looking at the bottom I saw how much board had to be removed. I carefully peeled back the leather cover, trimmed the board, tapered the board to a knife edge, and then used a Dremel tool with sanding drum to grind away material from the back of the leather. I then used blue masking tape and taped the leather to the board and dry fitted the panel, the chrome finisher on the end of the door, and the "C" trim around the door latch. It took me 4 trys and about 3 hours but I have a door panel that fits about 90% as well as the factory did it. I have an untouched 280SL with perfect door panels and used it as a guide.
Do not forget to clean the glass runners and channels and lube. Also lube the window lift, install a new outside mirror at this time and check it fits 100% and adjusts before you install the door panel. Also install plastic sheet over the door frame to act as a vapor barrier. Sign you name to the inside of the door panel should you need to ID some obscure part of your car for theft recovery.