Will you be viewing from the viewing area (causeway) or have you received a pass to one of the areas inside "the fence"? I think you need tickets for the causeway area.
From the viewing area, it's possible 300mm will not be enough...or produce a very small image of the vehicle. It's been awhile since I've been there but if I remember correctly, the causeway is about 6 miles from the pad (for security and safety). The other areas are 10+ miles away.
If you have a larger megapixel camera, you can crop to get a better image.
Also, it will be down range very quickly, so plan ahead or all you will see is a smoke trail!
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