Wash cucumbers, drain, and remove 1/8 inch off the blossom ends.
Dissolve the ¾ cup of salt in water in a large bowl or non-reactive pot. Add cucumbers. Add enough water to cover cucumbers. Cover and let sit overnight.
Drain and rinse cucumbers. Slice each one in half lengthwise.
Tie mixed pickling spice in a spice bag.
Pack jars with halved cucumbers. Add 1 teaspoon mustard seed and 1 teaspoon dill seed to each jar.
Heat vinegar, sugar, ¼ cup salt, water, and bag of mixed pickling spices to a boil. Reduce heat and simmer for 15 minutes.
Remove spice bag. Ladle vinegar mixture over cucumbers in jars, leaving ½ inch headspace. Remove air bubbles, wipe jar rims with a damp paper towel, and cover with two-piece lids.
Lower jars into a nearly boiling water bath canner, making sure that the water level is 2 inches above the jar lids.
Process for 15 minutes. Let sit in canner for 5 minutes,then remove jars and cool for 12 hours.
Makes 4 quarts of pickles.