You have a few options: One is you can just replace the 3rd member, it will bolt right into the 85 housing.
Other option is to replace the whole axle, it will bolt right up. And to widen the front you can either run wheel spacers, or get some IFS hubs and swap them on, which takes a little creative work, but plenty have done it, and I plan on doing it when I solid axle my 92.
The housings them selves are the same, just one is wider to make up for the additional with of the IFS. However, you said you got all your parts of a v6, the v6 3rd itself is stronger with bigger bearings and more cooling fins on the housing with a 4 pinion carrier (4cly has a 2 pinion carrier), but the v6 3rd will bolt right into the 4cly housing.
And you get special bonus points for bumping an old thread vs starting new one. :bravo: