Swapping fresh rebuilt heads onto a shortblock with unknown mileage/ condition doesn't guarantee the same results as an entire rebuilt engine- your power output ultimately depends on piston ring condition, and how well they seal. The block surface should also be thoroughly cleaned of all gasket material and checked with a straightedge to make sure it's flat enough for a good head gasket seal.
If you pull your 6x-4's and find a ridge at the top of the cylinder and taper in the cylinder bores greater than .005", your power gain with those small chamber heads will be short lived- eventually your rings will wear out, seal less, and allow gases to blow by the rings. Ideally you want your cylinder bores to be the same diameter at the top and bottom, so the rings seal consistently and last longer. A taper forms at the top because compression pushes on the rings and the rings dig harder into the cylinder walls at the top than they do at the bottom.
If you pull your heads and there's no ridge but the cylinder walls are smooth, you may consider pulling the shortblock, ball honing the block and fitting new rings to the existing pistons (pistons should be cleaned of course before you install the new rings). The honing gives the new rings a rougher surface to bite into and provide a better seal. It's more work, but the results will be better and your power increase will last longer (25-50k mi. depending on how you drive).
Geno
