I'm sure you know that higher capacity axles won't change your tongue weight, so that will do nothing to improve the way the trailer handles. I think to make this trailer safe, and the way that you're planning to, you need to move the axles. As for not building a new frame, I think it would be absolutely necessary to beef up the enclosed trailers frame. The trailer was engineered to be a bumper pull trailer, with X amount of tongue weight and X amount of load capacity, you're proposing to drastically change both of those. As for sliding a tube inside the original frame: that idea sounds great, in theory. The problem is for it to really change the strength of the frame it would have to be a very tight fit. But that won't really be possible because of the weld bead that is on the inside of the tubing. There are ways to clean that bead up, but they would require an investment in special tools, and typically require both ends of the tubing to be exposed, and I assume that isn't the case here. Like others have said, I think you really are going to come out ahead selling your enclosed and dump insert and buying a professionally built trailer. Have you considered that if you are ever in an accident with this trailer and the insurance company (yours or otherwise) finds out that you (neither a welder or engineer, as far as I know) designed and built this trailer they won't likely cover ANY claims. Even if the trailer design and engineering played no part in the accident. I don't mean to be critical, and I would really love to watch the progression of this trailer, it just has bad idea written all over it.