Hercules pixels aren't square.  To get a nicer "aspect ratio", you'll
probably want to stretch the x axis by 1.625, or to stretch the y axis
by 1/1.625.

If you happen to have something you're shrinking to fit the 720x348, you
might want to consider that you'll end up with a pleasant aspect ratio,
so long as the xscale-ing over the yscale-ing, yield 1.625.  For instance,
I've displayed a 290x583 gif, shrinking y, and shrinking x a bit less, just
enough to fit, with very nearly minimal loss of precision, with the
following filter:

giftoppm < fred.gif | \
	pnmscale -yscale 0.5969 -xscale 0.9699 | \
	ppmtopgm | \
	pgmtopbm | \
	pbmonherc 

You can leave out the backslash & newlines, of course.  I was just making
it fit on one line.

Enjoy.

- Dan