DAY BEFORE CANNING: Cut the pumpkin into large chunks, discarding the stems, seeds, and stringy innards attached to them. Pare the skin from the chunks. Slice each chunk into pieces 1/2" to 3/4" square, as evenly as you can. Stir to combine the pumpkin pieces in a large glass or ceramic bowl with the sugar, lemon juice and zest, and 1/2 cup water. Cover and let the mixture sit overnight, or a few hours longer, at room temperature. It will give off a good bit of liquid.
DAY OF CANNING: Prepare 5 or 6 half-pint canning jars by washing thoroughly. Place the jars on a rack in a boiling water canner, but DO NOT add water yet!
Spoon the pumpkin mixture into a large heavy saucepan and bring it to a boil over medium-high heat. Reduce the heat to a bare simmer and cook, covered, until the pumpkin pieces are quite tender, but still hold their shape and have become somewhat translucent in a glaze of thick syrup, about 30 minutes.
As soon as you have reduced the heat for the pumpkin mixture, fill the canner with water; water should be at least 1 inch above the tops of the jars. You may need to add 1 cup of white vinegar if you have hard water in your area. Boil the jars for 10 minutes to sterilize. It will take about 20--30 minutes for the water to come to a boil. Start timing from the time it reaches a good steady boil.
While pumpkin is cooking, blanch the lemon slices in a small saucepan of boiling water, just enough to soften them, and drain. Stir the lemon slices into the chips just before removing the preserves from the heat.
Once the jars have sterilized and the pumpkin is ready, remove jars from canner to a surface covered in heavy towels or layers of newspaper. (NEVER place hot jars on a bare countertop, as the difference in temperature could cause jars to shatter!) Spoon the chips into the hot jars, leaving at least 1/4-inch head space.
Process the jars in a boiling water canner for 10 minutes. Refrigerate jars after opening.