You said it blew your water pump gasket too? When you say overflow tube, are you talking about the puke tube from the radiator neck to overflow can? A couple other engine builders here have said don't stop with a mag check for cracks-ask them to do an actual pressure test. And just because you don't see anything physically wrong with the gaskets doesn't really mean they aren't sealing 100%...
When you pulled the intake was there coolant/water in the ports? Carb? Intake runners?
My own experience with magnum swaps...I tried to use the .028" gaskets on my 5.2 build and it didn't take well...it started leaking coolant into the oil as soon as I fired it up, and it let some out of the pipes too, but with an X pipe, I couldn't narrow it down to a side, so I just replaced both. I went with the Fel-pro magnum gaskets (which are probably .045") and just rolled with it. At the time, I figured there was just enough warpage/planar issues on the decks/heads that the .028" gaskets just couldn't reconcile. After swapping to the thicker gaskets it's been fine...at least until I stored it about a year ago.
