Casting/stamping numbers. What do your eyes see?

I can't say I've ever seen a 340 with a rebuilders tag before... I'd say to check and see if it is actually .040 over. Anything over that in a post 1970 casting tends to be a thinner casting, so a sonic check would be in order before going .060 over. If the bores still look good with no obvious ridges and it has cast pistons, then there may be enough material to "hone to round" for a forged piston.