Just to let everyone know, my 91 Civic passed emissions with flying colors!
The new cat dropped my CO, hydrocarbon, and oxides of nitrogen down to
practically nothing. A dramatic difference! Even though carbon dioxide
levels don't count they do measure them and they were way down as well.

Hopefully I can drive the Civic until my van is paid off. My daughter
is 13 and it would be a great car for her to learn with.

Just an aside...even though emissions testing is a pain I wholeheartedly
support the program. I think Maryland's plan is fair - $14 for the first
test, if you pass you're good for two years. If you fail the second test is
free and they give you a list of troubleshooting ideas. Subsequent tests
are $14 and they end up giving you something like 6-8 months to correct the
problem. If it still won't pass you can get a 2 year waiver if you can show
repair receipts for at least $450. Sure it costs money, but if you pass
(like most people do) it's $14 every 2 years. A paltry sum when you
consider the benefit to the environment. If you fail you may have to spend
a few bucks, in my case about $350. But I did get a tuneup and a new cat
out of the deal. My car's running great and I've minimized my contribution
to air pollution. Win-win.

Thanks again for all the help from the folks here at alt.autos.honda!