Its hard to say what is actually siezed...is the bolt seized in the threads, or is the bolt shank seized inside the sleeve of the mount. It sounds like you have just installed a new mount, in such case the bolt would not be seized inside the mount, but rather in the threads. Without knowing, here are some pointers, some of which may apply to your situation:

1) Bottom line, I think you need to get the whole business out (bolt, mount, etc.), then inspect, clean, etc then re-install

2) If this is an old mount and unattended or a long time, then the bolt shank may be seized (rusted) inside the mount, in such case you will need to burn it out...apply heat (propane torch) to the bolt until the whole mount burns, then just pry out the pieces, and start over. The reason for this is that with the bolt rusted to the sleeve, when you try to unscrew the bolt, the whole rubber portion of the mount gives, and you cant get solid purchase on the bolt against the sleeve. You need to fry/burn the rubber with a torch, remove the bolt, then knock off the remaining sleeve with the bolt in a vise. To re-use the bolt, clean/sand/file off the corrosion, and slather on anti seize so it wont seize in the new mount.

3) When installing a new mount, you can't seat the mount using the center bolt..the rubber portion of the mount will "bottom out" before the mount is fully seated. Plus the bolt is not long enough to catch the threads with the mount protruding. You need a shop made tool (I made one out of oak and threaded rod) to pull the mount in by applying force to the rigid sides of the mount...you also need to apply silicone grease to allow the mount to slide in. When the mount is fully seated in this manner, THEN install the center bolt. I am worried that you used a different longer bolt...was it the same thread size? If not, this is obviously the problem. No matter what, you need to remove this bolt.

4) If the bolt is cross threaded, you simply need to take it out in any way you can (breaker bar), and remove the mount, then try to re-install only the bolt to try to properly install the bolt and "chase" the threads...I think it is unlikely that you cross threaded the original bolt, because that bolt is "self aligning and self threading", although with a different bolt cross threading is not out of the question, especially if thread size of the new bolt is incorrect.

I hope this helps...

Mark

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)