Wash cucumbers; remove a 1/16-inch slice from the blossom end and leave about 1/4 inch of stem.
Dissolve 3/4 cup salt in 2 gallons water, pour over cucumbers, let stand 12 hours, then drain.
Combine 5% vinegar, remaining 1/2 cup salt, sugar, and 2 quarts water. Add mixed pickling spice in a clean cloth and heat to boiling.
Fill jars with cucumbers; add mustard seed and dill as directed by the source.
Cover with boiling pickling solution, leaving 1/2-inch headspace. Adjust lids and process by altitude.