Your 870 is perfectly appropriate for Bull Run, though members of the Perazzi crowd may not think so.
I'm not sure if they have any barrel length requirement, though. I've used my Mossy 500 before with a 28" barrel (now out of commission), but not with the current 18" barrel. Bull Run has really nice autos and O/Us for rent.
I like Light and Xtra-Light target loads. I can shoot those all day, but after a handful of Fiocchi slugs, I'm ready to pack up the gun and go home.
If you want to meet up on Sunday, I'm free.