If you do your measurements correct, yes the inner shaft will fit into the diff and the the 45 birf. I would go with the larger bolt pattern on the later series knuckle (mini truck and 60 series). Tons of options for steering arms. '78 and down knuckles have a smaller pattern. For that...
simple answer is Yes you can widen it. There used to be a kit to do ~3" to one side, sure you could add it to both sides. Might be better off ordering a custom housing that will be much stronger. 80 shafts should fit the 3rd you have. Length might be another question. Why not use a 80 front?