Exploding water pumps? Difference in early 273 water pumps vs later 1970s 318 pumps

My 67 oem setup. 2 1/2" spacer 7 blade fan and shroud. Waterpump is a cast iron repop. Had to machine pads/bosses on alt side. Machining is/was off.
Part of your problem I believe is the magnum heads maybe?

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