If it were my engine i would have the heads milled down to a smaller chamber size. If you've already spent the money on aftermarket heads in the first place it's worth the cost of machining to see if that helps.
Just for the sake of correctness i think that with a 60 over bore you'll actually be at 388 cubic inches. It's always nice to know that you've picked up an extra 5 cubic inches though
double_v, is that like an industry standard for piston manufacturers? I'm just curious if that's what you should expect the rating to be given at for any company. That's pretty good information if it is!