Watch your wall thickness. At 0.245 piston bore difference, you will be in the water jackets.
Pontiac blocks are all the same dimensionally 326-455 but internals differ.
You could put 400 pistons in a 350 but will more than likely need to be sleeved. I would start w/ a 400 block. I have a pair if you are interested. Shipping over the boarder would be costly though.
Wallace racing has lots of quick reference facts
http://www.wallaceracing.com/boresizes.htm