I'd say go for it. They should be close enough to be an easy job to modify further. Best case scene: perfect fit, no cutting/welding. Worst case: cutting and modding several tubes to make it fit right. $$ spent will be $$ spent.
If you want fenderwell headers you have two choices. Buy the ones you mentioned and take your chances, or completely fab your own.
My choice? Buy the ones you listed and mod them if needed.
And make sure you get plenty of pics afterwards too! There's some interest in B/RB Dakotas, so your pics would really help the other peeps doing that kind of project.
I have seen it done. The guy beat the shit out of them to get them to clear. To the point he beat one primary half-way closed. He was running a 15in tire but I can't tell you what size. He could turn nowhere near full lock without the tires hitting the headers. It was a "race truck" only but it was pretty ragged looking. (note: I only say they were A body headers because thats what he said they were when I asked)