I'd think it would depend on how you plan on extending the arms, if you're planning to cut the stud off the old arms and re-locate it to the end of the extended arm or if you plan to use like a Johnny Joint.
If you are planning to re-use the stud, you might be able to make something work if you used the radius arm mounts from an F-150. I know when I replaced those mounts on my F-150 I held the old mount up on my choptop and it looked like it could work.
I did custom extended radius arms on my choptop last year and made my own frame mounts for using a like Johnny Joint. I ended up going with the Ballistic joint (from Ballistic Fab) because I wanted something made in USA. For the mount on the frame, I used a piece of 1/4" plate and a piece of 3x3x.250 angle.
Ignore the "left" mark on the steel, that is really the bracket for the right hand side.