From this particular source, pistons are offered in .020, .030, .040 and .060.
http://webpages.charter.net/beckracing/slvpg56.htm
My personal preference would be to use the smallest oversize which will clean up the existing bores.
First, you will need to mike the bores to see if the engine has been rebored before. You shouldn't assume that the bores are standard size now.
Ray