If you look through posts on the trailers thread you will find that many say its cheaper to have a custom trailer built than modify a boat trailer. They are probably correct unless you have the know-how and a tools to do the job yourself (or a good friend with the aforementioned skills and tools). That said, I built mine on a used boat trailer... my brother is a welder/fabricator and he did the work for some woodworking on my part. The modifications consisted of shortening the trailer, adding an A-frame tongue, and relocating a cross member to make room for the dropped floor. No change in width or axle placement. I built out over the sides so the wheels are half under the frame, half exposed.

If I remember correctly most of the shorelandr's have the v-shaped center members. I have seen where some have built the floor up but you are adding a lot of weight. If you need to replace them you might have a lot of work to make a usable frame.
Remember, many boat trailers are built much heavier than needed for a teardrop.
Good luck with your build and keep us informed about how its going.
Chuck