Thanks to everyone for the replies.

If cutting into the wall is going to leave an ugly opening, then I suppose I
could live with the ugly added plumbing needed to install the single lever
valve, or I could always just go with two individual ball valves if that
would look better and avoid cutting into the wall.

Too bad I wasted time saturday afternoon trying to fix the drip in the
existing crank-style valve by attempting replace the packing, but something
is very peculiar about the existing one in which the packing nut, no matter
what I did, always seemed to go back onto the threaded cylinder piece at an
angle. The nut does stays on, but since it goes on at an angle I don't like
that I can't tighten beyond a certain point without fear of stripping the
threads....don't know if it was me that did something wrong or there is just
something peculiar about this valve already. I guess I'm better off
getting it replaced by a professional rather than wasting more time with it.

If you can live with it looking half assed, then the existing valves
can be unsoldered, some quick plumbing added to bring the lines closer
and line them up with the new valve. That should cost a lot less
than the $350, but it leaves ugly and mis-aligned pipes exposed, which
may or may not matter to you.

If you want it done right, that would be to open the wall and put in a
valve box. For that $350 may be about right.