You should have a shock stud or some spacer or slightly canted mount on the bottom. That will give you some clearance between the shock body and the rear end housing & that will get you that much closer to the rear. Putting a long bolt thru the hole is not the best idea but it should work. Just grease it well so you can get it out. Also some kinda gusset would be a good move if it needs to be stuck out that far. Single shear mounts should be on a shock stud. Where's the springs?
Another thing to consider, make sure your axle bottoms out on the frame before your shock runs out of travel. Otherwise you will break the shock mount. Ago
The spring is behind the axle. I was thinking about making a spacer for the bottom mount. That would bring he shock closer to the rear cross member.
Ok, so you aren't locked into anything yet. Rotate the shock mount - bottom towards the rear crossmember . Pie cut the mount to bring stud back to plumb. Now you just easily split the difference.