Tie pickling spice in a square of cheesecloth to create spice bag.
In large stainless steel saucepan, combine vinegar, water, sugar, pickling salt, and spice bag. Bring to boil over med-high heat, stirring to dissolve sugar and salt. Reduce heat and boil gently for 15 minutes, until spices have infused liquid.
Place 1 bay leaf, 1/4 tsp. black peppercorns, 1 clove garlic, 1/2 tsp. mustard seeds, and 2 heads of dill into each jar. Pack cucumber slices into hot jars to within a generous 1/2" of top of jar. Ladel hot pickling liquid into jar to cover cucumbers, leaving 1/2" headspace. Remove air bubbles and adjust headspace, if necessary by adding hot liquid. Wipe rims. Center lid on jar. Screw band down until resistence is met, then increase to finger-tip tight.
Place jars in canner, ensuring they are completely covered with water. Bring to a boil and process for 15 minutes. Turn off and remove canner lid. Wait 5 minutes, then remove jars, cool and store.