This is where a dremel with a grinding stone comes in handy! I'd be concerned about wallowing the hole out in one direction by the time it was big enough to fit the bolt through if just attacking it to round it out with a drill bit, but in this application it might not be too big of an issue. A file would allow a more precise approach if you don't want to buy a bit, but if you don't have a 5/8 bit, a dremel, or a file, then I guess it's either go buy something or make do with what you have, depending on your comfort level.

 

Thank you for the advice!! I ended up just rounding the hole out large enough to fit my 5/8ths - 3" bolt through, then used a bushing and a washer to keep it snug to the wall of the shock mount. There is absolutely no play with the shock on the new mount- feel pretty proud I was able to fab something up!

I didn't continue changing the shocks due to rust on the top mount bolts- I was afraid I would break those off as well and didnt want to have to fab something up for that as well. I am thinking about using my MAP propane and heating the top mounting nuts for the rear shocks...and hopefully they loosen up- but obviously I am afraid of breaking the nut again...

Any advice to combat this issue?

 

Soak the upper bolts everyday for a week with pb blaster or equivalent before trying to loosen them.

Since you did a good job on that bottom shock mount, why not try this if any break. if you do not have a air hammer Im sure drilling the nut out will work.