Value of Slant Six Car

I'm going to swap out the /6 in my Duster for a BB some day. The 73 is keeping it's /6 though, need something cool to drive that won't kill me on gas. I'd say the 4dr is worth about $400 without knowing the extent of the rust. I paid $300 for my 2dr 73, the frame is solid on it, however it has rust in the floors, 1/4's, rockers, around the rear glass, etc. I like it though.