Ayuh,..... It sounds like the inner liner of the rubber bushin' is rusted solid to the bolt,.....
Yer gonna need new rear motor mounts,....
You could burn out the rubber, 'n pull the bolt out with the rusted collar still on it,....
Be careful, 'n have have fire control close at hand,....
Donno if you could even get at it with a sawsall,....??