248 block with 3.905" bore and 3.625" stroke = 260.49".
IMO this is faster than a 248 bored to the same displacement: a few more RPM, but more important the big bore un-masks the valves, and a far lighter piston.
Using the Pontiac rod, the CD is about 1.719" for zero deck.
The only thing I found (quick search) is a cast 350 Pontiac piston at 3.875" + O/S, has 1.710" CD, flat dome with small valve notches, .9805" pin, available from KB: http://tinyurl.com/yg444fh