did you straight edge your cyclinder head ? was it brand new ? either way i have had new heads that are ever so slightly out of true from new !! and a second hand one will definately have been warped .
assuming it is true and straight , i have had 3 k series engines with dropped liners in the past so i think that is a definate possibility
i have never heard of a cracked liner on a standard engine only on heavily tuned k series engines , you could pressurise the cooling system and then remove the plugs and put the pistons to the bottom of each cyclinder and look inside for signs of water , but this would not show a leak on the bottom of the liner itself