Do what I did, and they will come out, u might strip the bushing or bolt. But mine eventually came out with a Breaker bar.

 

The front body mount top bushings/nuts on my truck were also seized, I found the easiest way to remove them is to use an angle grinder and cut off the bolt heads, then you can pull out the rest of the bolt with the bushing/nut attached. I put that into a vise and was able to remove the rest of the bolt from the nut. Since I was adding a lift, I didn't care about the bolt, but if you aren't adding a lift, you can still follow the approach used by the lift suppliers:

• Us a 1/2" drill bit and remove the thread from the nut,
• get 2 grade 8 bolts the same length as the old metric bolts (actually, get them an 1" longer & get the grade 8 nuts to go with them).
• You can re-use the thick washers for the bottom, then slide the new grade 8, 1/2" SAE bolts through, put the bushing on (it should just slide right over the bolt now that the threads are drilled out).
• Add the 1/2" grade 8 bolt & tighten it all together - very easy to do and a lot cheaper than getting the metric stuff from the dealership