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

To the OP. In theory yes you could just deglaze the block and install a different set of pistons. The standard recommended clearance is built in to the piston IE: 4.03 bore piston with recommended .008" clearance the piston is 4.022" now of course there are tolerances to take into consideration but for a inexpencive replacement engine like a crate motor they are not mesureing every piston and matching it to a particular bore that is only done on all out race engines. so depending on what you intend to use the motor for is what you have to decide. from the sounds of it your just looking for more compression for a fun motor. as long as the pistons are weight matched to each other as in most sets of KB pistons sold as a set are you could just hang them on the rods and gap the rings for the KB pistons I prefer SP hypers no special gap and run them.