Once this is done, check the head gasket for any flaws before assembly - especially the seal around each cylinder. Make certain to follow the GM head bolt torquing sequence explicitly, any deviation can cause the head to 'bow' and not get the proper force between the block and head at the cylinder bore 'siamese' sections (the narrow sections of the adjacent cylinders).
One old school trick to get the head gasket and head to line up correctly is to use a couple long bolts (with the same thread and at least an inch longer than the head bolts), cut the hex off the top of the longer bolts (chamfer the square edges of the cutoff), and cut/grind a slot in the remaining shaft so a screwdriver fits well. Hand thread them into the block at opposite corners, before any assembly, to have alignment studs. Drop the head gasket in place (should be perfectly aligned to the cylinder bores) and then the head - which should also be perfectly aligned too. Unscrew the alignment studs and install the head bolts.