You can also leave the cockpit back section you have now and chop it off a little forward of the sides and add on the section you need. Sounds you're not a woodworker? Just use a jig saw from the inside (carefully) and saw it off paying attention to be a tiny bit above the deck so you don't scratch it. With chisels, scraper / sanding etc. it will be easy. You also might be able to bust if off with a chisel striking from the outside with some relief cuts so it comes off in pieces.