I had a similar issue on Dad's 65 Barracuda, the previous owner had used a DA on it 30 years ago. It was horrible.

I used a Ditzler metal prep, that you mix with water and wipe it down. It turned the rusty metal a blue color, similar to metal that had a torch put to it. Rust was gone! I am not at home, but if you are interested, I will get the part number.

*edit* I did remove as much as possible, prior to using the stuff