Magna-fluxing found a Crack in one of the heads

That casting number is shown to be both 318 and 360 castings from 1977-1982. Available with the 1.78 (318 ) and 1.88 (360) valve sizes. Are you sure they are 360 heads? Because if they are the 1.78 318 heads, they are not even worth taking to the shop in the first place.