You might have to contact Catalina Yachts directly to see if you can get the extrusion in longer lengths.  I had no luck with Taco Marine.  I ended up buying the 6-ft sections from CD.  I found it was much easier to work with these smaller sections anyway, and absolutely no one will notice that the extrusion is in sections.  The rubber insert is one long continuous piece, and that's the part that makes the rail look good anyway. 

The screws and bolts are readily accessible behind some trim pieces in the cabin, so you'll have no issues removing the old ones.  However, the existing screw holes in the fiberglass will NOT line up with the holes in your new extrusion.  I have read on here that one solution to this is to remove the old rail, put the screws back into the fiberglass, put the new rail on over top, and drill large holes into the new extrusion to fit over top of the old screws (where needed - some of the screws are countersunk), and then drill new holes through the extrusion and fiberglass to attach the new rail.  I opted instead to just fill the old screw holes in with epoxy and drill new ones.  This made the job take a day longer waiting for the epoxy to fully cure, but it seemed easier to me.  

Getting the rubber insert into the new extrusion can be a nightmare (I did the job by myself), but doing it on a warm day helps.  I have read multiple ways of doing this - rubber mallets, flat screwdrivers, soapy water - all worth trying.  My favorite advice was to soak the rubber insert in hot water and then feed it from the water into the extrusion.  I'm not sure where people get hot water in a boatyard in spring.  Maybe they have a pot sitting on a barbecue grill or something.  Once you start hanging the rubber insert, you'll see how impractical this advice is, IMO.  I found soapy water to be the most helpful for me.  Good luck!