if I remember correctly there are 2 threaded plugs in the back of the head, each of them is 1/4" NPT(I actually used to use one to feed my turbo) and then there are the standard freeze plugs at the end of the head, dont remember how many though...I never actually replaced mine(I only replaced the ones in the block and forgot the ones in the head) but 5/8" sounds about right for the press in type ones...maybe a little bit smaller