I would start with a PG 670. If you shoot it and it doesn't work, you can get your money back. If that doesn't work out you could try and Jelly Head 660-665. They seem to work for most people.
You can also give Sumtoy a call. I haven't had the pleasure of dealing with them, but I have heard great things.
I have a PG in my 3" 870 and run mag blends through it. I am getting over 200 consistantly.