Kevin,
In reality, boat trailers are really pretty simple items, and having new frame rails fabbed up, is not that big of a deal, especially if everything bolts together, you are not trying to make it wider, just simply longer to accommodate this boat, if you don't want to weld and you don't want to lengthen, then that leaves pretty much one thing, have new frame rails made and then bolt it all back together and be on your way.