Once you get the body off, and based on how long you want your trailer to be, it should be obvious where to cut it, adding on the length for the new cross-brace triangle to the hitch. You could cut anywhere in front of the leaf spring mounts, except that you want to keep in mind what the load balance is going to be, not having it too heavy behind the axle, yet not too much tongue weight either.
Edit: I was under the assumption you meant you want a trailer to tow behind a truck or large car. If this is to pull yourself manually or pull with a small garden tractor, then it seems like an excessively heavy burden to move that much extra weight around.
How closely have you looked at it? I assume you are aware that it is not flat, arches up at the rear wheels.
I'm not 100% sure this next pic is from a 2nd gen but it's close if not.