Nah, I'd be shocked if it had anything to do with re-using the bolts. I've re-used them on both my 602 and 603. You have to abuse the bolts pretty bad to stretch them beyond spec. I have replaced a couple with boogered-up 12-pt heads though. More likely to be a head gasket, or freak head failure. I'm very interested to hear what you find when it comes apart - please post a link to your thread, assuming you start one.

I assume you used a name-brand gasket set, chased all 27 or so head bolt threads with tap & die (both the bolt AND the block threads), and soaked all liquid out of the block? And used clean engine oil on the threads when installing? And used the factory 4-stage torque procedure with an angle rotation gauge, not eyeballing it? Lots of little things to go wrong if you're not careful...