A friend blew up his stock 502 with a whipple.If the block is OK, instead of just rebuilding it, is there an upgrade kit like re-program,pulley size, pistons? has anybody been down that road? what kind of HP can we expect?

You can talk directly to Dustin at Whipple, they have spec'd and done a lot of 502-540 BBC engines for their superchargers and they probably know the exact recommendations for pistons, compression, head work etc and changes in programming, pulley, etc.

In any event use a qualified marine engine shop to redo the block and don't let your friend do it out of a catalogue on his work bench in his his home garage type scenario. Good reliable, long lived supercharged high horsepower marine performance engines need to be done by extremely qualified individuals or shops with lot s of special machining equipment and expertise. We have a lot of good shops all around the country who are right here on OSO who can help your friend "get it right"!

Best Regards,
Ray @ Raylar