1. Wash cucumbers, and trim any that are longer than 4 inches (so that they'll fit comfortably in the jar). Cut each cucumber lengthwise into quarters. Place spears in a large clean container (such as a 12- to 18-qt. plastic pail or dish basin). Combine 6 Tbsp. salt and 1 gal. water in a large pitcher, stirring until salt dissolves. Pour over cucumbers; cover and let stand at room temperature 24 hours. Drain.
2. Sterilize jars, and prepare lids.
3. While jars are boiling, combine vinegar, next 2 ingredients, remaining 1/4 cup salt, and 1 qt. water in a 3-qt. stainless steel saucepan. Bring to a boil.
4. Place 2 dill sprigs and 1 tsp. mustard seeds in each hot jar. Pack jars tightly with cucumber spears. Cover spears with hot pickling liquid, leaving 1/2-inch headspace. Seal and process jars, processing 10 minutes.
5. Remove jars from water, and let stand, undisturbed, at room temperature 24 hours. To check seals, remove the bands, and press down on the center of each lid. If the lid doesn't move, the jar is sealed. If the lid depresses and pops up again, the jar is not sealed. Store properly sealed jars in a cool, dark place up to 1 year. Refrigerate after opening.
Note: Pickling cucumbers are small, crisp, unwaxed, and needn't be peeled. Widemouthed jars aren't essential for pickles, but they do make for easier packing.
Total time: 50 min., plus 1 day brining and 3 weeks standing time