Hmm... You know, what you really want to do is tear the whole thing to pieces and rebuild it again. It has different types of pistons, even some with dishes and some without. That thing is never going to run nicely, no matter what you do to the heads. The chap who built it didn't know really what he was doing (putting different pistons in an engine ) Maybe your heads weren't torqued up properly and lifting the engine (if you lifted it by some head screw holes) just dislodged it by a couple of thousandths, thus breaking the seal of the gasket. The compression and burning proces then did the rest. Maybe the dowels arent far enough in the block, then the heads wouldn't rest properly and your 4 corner cylinders could spring a leak
I don't think the valves hit the gasket, the gasket seems to go round outside the bore, and a valve can't really hit the bore, unless there's something really wierd wrong.
I would tear it down, get some hyper pistons, pop them in, have the block cleaned, honed (maybe rebored, measure it first) and surfaced, a bent head might have bent your block. Resurface the heads, too. But check, that you don't run into clearance probs.
If you need more help, here's where to get it.
But as always, that's just my way of thinking.

If you chose to just set up the heads again and pop them on, then be prepared to tear it down again soon. It might run nicely for 50k miles, but on the other hand, it probably won't.

That's all from me,
Max