I don't have nay problems with drilling them, if you feel that it's warranted, it won't hurt anything. But personally, if the factory design didn't have it, it really isn't necessary. Proper filling and purging of the system should take care of any air in the system.
Tim, I'll have to look at the '60 manual to see if it was factory or not. I do know that the cooling system is different due to the bypass. I think '59 was also done like that on the 261.
