When I made the Voyageur longer it was my first packraft build and I screwed some stuff up. If I had to do it over again, I’d say no, it is not harder, just more time consuming by adding a couple seams.
I can’t answer your question about self-bailer. If you buy a Big Agnes, be sure it fits your floor length. My Voyageur is gigantic, and it fits perfect and works really well.