I've bored out a later model 413 truck block to 426. Stuffed in some max wedge pistons I had lying around, haven't had any problems yet. It is a bit of metal to take out, but remember, that's how they made the 426 in the first place. And from what i've seen, good 426 pistons are easier to find than 413 pistons.
For what its worth though, I did all the machine work myself, so I know it was done right. I wouldn't let anyone less than reputable take that much out of an engine.
