the hardware store has a 5/8 by 17" bit for ~ $20 & that is enough length so I can make some kind of support so it drills straight. thanks for the heads up on the extra fitment needed & I read one time a guy was peening it with the same type MP tool I have & he broke the lifter wall so yes its gonna take some patience (I have that) & after 4 head gasket failures immediately upon fireup on THIS particular eng its gotta be right this time!
RR, make sure your fixturing is as ridged as you can make it, because any flex induced by it will be compounded by the length of the drill, which will flex some, even though it is 5/8", and you wouldn't think it would. if it were me, i would drill with a 39/64 [.609] drill, then ream with a 5/8" reamer, which would guarantee you a 5/8" hole top to bottom. it's very likely just using a drill of that size will result in a tapered hole just because of drill flex due to the extended length. but that would increase your tooling cost to approximately $50-60.00.[just a guess]