For the Spam:
In a meat grinder fitted with a ⅜-inch die, process 35 percent of the pork shoulder. Grind the remaining meat through a small die. In a stand mixer fitted with the paddle attachment, combine the meats with salt, sugar, cure, pepper, garlic, allspice, cinnamon, and mirin. The meat must be as cold as possible, preferably below 35°F. Mix meat to combine, about 2 to 3 minutes. When meat mixture is homogeneous and appears sticky, begin to add ice water gradually. Test for seasoning.
Place mixture in a terrine mold in small handfuls to avoid any lumps. Let stand in the refrigerator overnight. The next day bring to room temperature. Heat oven 200°F. Prepare a water bath, place terrine mold inside, and cook Spam until internal temperature reaches 150°F.
Remove terrine from oven, and chill 1 to 2 days before serving.