First, you can look at the shorter versions and just build one like it with the proper spans. There are great new boards out there with all the necessary cuts and grooves to make your own from scratch.
Secondly and my concern is that the spans between posts are too far apart for railings without posts between the spans. One lean on a new rail that only has posts or supports every 12 to 14 feet would render the person breaking through and causing many problems.
So, I would still build my custom longer span railings by scratch, then figure out a way to have supports or posts every 6 feet or so.
I know that this is not what you wanted to hear, but its best to hear it before work has begun than after.