The screws in the carry over tube have fused how can I get them off (unscrewed) without breaking something?

Great question! Unfortunately you will not be able to get them out without breaking something. Fortunately, the thing you will probably be breaking is the rusted screws themselves, and then you can replace them with stainless steel bolts and nuts. Try using WD-40 first and if that doesn't work to get the old ones out, break, cut, or drill them out.