The Crown Vic subframe has a wider track then a Dakota, which becomes an issue on many older cars & trucks. Around here its easier and cheaper to buy a running Dakota then it is to buy just a frame, but yes, you will want to look at the Dakota frame for rust before you part with your money. Areas of concern are mostly between the wheels on the lower part, especially near the trans crossmember and the joint area near the channel & the box joint. Don't completely condemn the frame if there is rust in those areas, if the rest of the frame is pretty clean. The bottom sections can be replaced with 1/8" flat stock. Your probably welding on the frame anyway. Like most modern stuff, they created a location where water can get in, but can't get out, so rust begins and self feeds. Gene