Can I swap one set of .030 over pistons for another, without machine work?

KB hypers want a tight piston to wall. Most any other stock type will want a wider piston to wall. Fatal issue? No. But it's not the best case and IMO you're better with what you have with low static than stick in pistons just for a point or two in compression. You don't say what year the 318 is,but if it's pressed pin you have to have them pressed off, the new ones put back in place, you have to balance it (you noted that already), and you have to give the new rings a surface to seat to. At minimum. So is that all worth 2pts of compression? You would be better served to take that cash and put it into either a better set of closed chamber heads that give you some compression, and better flow, or just port the heads you have.