If I were u, I'd get it, strip it down completely and take the block/pistons etc to your engineering shop. Has he mentioned if there's much of a ridge at the top of the bores? If there's almost nothing, possibly a hone & new set o' rings will sort it. Difficult really. Just watch out for deep scratches in the bores.
I had this dilema when I was going to rebuild my XE. The piston knocked around in the bore like nobodies business. Never heard any piston slap though.
There's more knowledgeable peeps here but to me that says dead rings or excessive bore wear (the ridge).
I'm also not sure how much 'play' there should be. Aparently there should be more side-to-side movement than forwards-backwards, but still, only a little.
Don't forget though, a re-bore means new pistons & rings