Set belly skin side down; with a sharp knife, score belly flesh in a checkerboard pattern 1/3" deep to enable even cooking
Flip belly skin side up; poke dozens of 1/8"-deep holes through skin all over belly (when you think it is enough, do more)
Optional: using the jagged edge of a meat mallet, pound skin all over for 3 minutes to tenderize, which will help make skin crispy when roasted.
Turn belly to be skin side down; generously salt both it
Season loin; sprinkle inside of each with 30g or Vin Santo Wine
Arrange loin down middle of belly; top with stuffing
Roll belly around loin; tie crosswise with kitchen twine at 1/2" intervals, very tightly
Trim twine; transfer roast to a wire rack set in a rimmed baking sheet
Preheat oven to 500°; season porchetta with salt
Roast on rack in baking sheet, turning once at 15 minutes, for 30 to 40 minutes, until golden (the oven will be a huge mess, so best to do this step outside on the grill)
Reduce heat to 300° and continue roasting, rotating the pan and turning porchetta every 20 minutes for one more hour, or until an instant-read thermometer inserted into center of meat registers 145°
If skin is not yet deep brown and crisp, increase heat to 500° and roast for 10-15 minutes more
Rest for 30 minutes; sing a serrated knife, slice into 1/2" rounds