If your pushrod hole is already 1/2"(measure them) 5/8" shouldnt be an issue unless ported. Eigth inch over half inch is only one sixteenth (.0625") per side. As long as you dont get too thin on port sides your good. Breaking out on side away from port is no issue. Shouldnt be any issue on water jacket or valley side.
fwiw Stock 6x heads and 4x I have are 1/2" hole stock and mine could easily go more than 5/8 hole.
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