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Can't help you with the bolt size, thread or length, but the bolt threads normally into a nut and can be removed after the threads are disengaged.
But I suspect you are really wanting to know how to separate the two halves of the bushings, for which you need to create a pulling device to do so. Pounding the bolt head from inside the cab is futile and a recipe for broken things, like fingers and stuff.
Do a search on 'body bushing' and you will find lots of advice and solutions.
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