After a very confusing mistake, I've gotten this running.
I'm doing something a little different. I just want to try this on localhost, so I'm not doing any ARP spoofing. Instead, I'm just forwarding requests to port 80 to port 10000. Of course, this created a very confusing error. My test curl request was redirected to port 10000, and sslstrip got it. It then tried to connect to the server on port 80, which was then redirected to itself. It sat there making new threads until it dies. I was debugging python before I realized what was going on.
Anyway, here's my iptables command line.
sudo iptables -t nat -A OUTPUT -p tcp -m owner --uid-owner test --destination-port 80 -j REDIRECT --to-port 10000
The easiest way was to just make a user called test and filter only connection requests made by the test user.
I'm about to play with this further, I haven't even begun to start doing any ssl testing.