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

I wonder what stewpot of parts I lucked into when I just screwed my stuff together.......

But I see you are running a tiny fan pulley, looks to be extremely small. My guess is that the ratio is something like 8 to 5=60 effing percent overdrive!
that means your pump is spinning 60% faster than the crank. And that means by 5000 crank rpm, the pump would be doing over 8000. It wouldn't take much of an imbalance to shake it apart.
I can't imagine why you would run it like that.

Are you having over-heat issues? Those are usually easy to solve.