charley;
Look at Peggy Hall's notes/book first as Steve pointed out. Thank you as always Steve. Then look under the Hunter 260/270 owners manual pages 57 and 58 for additional information. This will be a good source to look at.
If you are going to install a head, you will need to cut holes in the hull but let me warn you, never in the water ballast section of the hull. If you are nearby a Hunter 260 with a marine head on, look at that install and follow it to the tee. The hull for the 26 and 260 are the same with the same cabin configuration for the head and rear berth area. The 20 gallon holding tank is usually located behind the rear berth port side. You will need to build a flat surface supported well enough for the holding tank. Once you have studied this and think you know what you are doing, get the cost of parts first and then ask yourself if you want to put this in as it is not easy. Some would be concerned but it has been done in the past and I was the first to do so and not once but a few times. Do not be afraid to ask questions. After all that, I will be glad to talk with you on installation.