Soak cucumbers in ice-cold water, 2 to 3 hours. Drain.

Inspect five 1-quart jars for cracks and rings for rust, discarding any defective ones. Immerse in simmering water until brine is ready. Wash new, unused lids and rings in warm soapy water.

Bring 7 cups water, vinegar, sugar, and pickling spices to a boil in a separate large pot, stirring occasionally until sugar dissolves.

Place about 3 dill sprigs, 1 tablespoon pickling salt, 2 garlic cloves, and 2 horseradish strips into each sterilized jar.

Trim ends from cucumbers; cut a slit in skin of each so brine can soak in and air can get out. Pack cucumbers into the prepared jars; pour boiling brine over top, filling to within 1/2 inch of tops. Run a clean knife or thin spatula around insides of jars to remove any air bubbles. Wipe rims with a moist paper towel to remove any residue. Top with lids and screw rings on tightly.

Place a rack in the bottom of a large stockpot; fill halfway with water. Bring to a boil and lower jars 2 inches apart into the boiling water using a holder. Pour in more boiling water to cover jars by at least 1 inch. Remove pot from the heat; process until water is cool.

Remove jars from the stockpot; rest, several inches apart, to cool. Press centers of lids with a finger to ensure lids do not move up or down. Refrigerate any unsealed jars. Remove rings for storage and store in a cool, dark area.