As I mentioned before, the worst thing to happen running 1958 rockers on a 1960 engine is over oiling. I have heard of many people using this set-up with no problem. The main problem I have seen is keeping the valve cover gasket from leaking.
I have a few factory drilled bolts, or they are easy to drill your self, but I doubt you really need one. The head would need to be off to install the plug in the block. I would need to check on the plug size.
PM me if you decide to use one.
