For the aluminum radiator tanks I'm making, the bases are frames cut
from 1/4-inch thick 5052 aluminum. They are about 23 inches long about
4.5 inches wide, with a cutout about 7/8" inside that.

The problem with both pieces is that the metal is curved slightly, so
that on a flat surface two opposite corners sit about 1/32" off the
surface.

I have read about peening the bowed-inward side, but the problem is that
the bowed-outward side needs to form a watertight seal, so I don't want
it marred up. I have some smooth pieces of steel around, but I'm
wondering if doing this on them will still mess up the surface of the
frames...

Without special equipment is there any other way worth considering to do
this? I have a 3-ton arbor press and a 20-ton H-frame hydro press, dunno
if they will be of any use or not.