360 head bolts
Best way to solve this mystery is to verify the head bolt holes are infact "blind". Long screw driver/piece of coat hanger works for this, hit bottom about 1 1/2 inches in it's blind, no need for sealer, just don't use too much oil on bolt threads as you could hydraulic the block and crack it, keeps going, it's a thru hole into water; seal or not for your own peace of mind.
Easy-pezie.
If you had chased the threads in the block with a tap, you would have discovered if the holes were blind or thru.
Good luck!