Jimbo,
I had planned the same thing, then bought a 5' wide tear I'm restoring. I'm not sure if you have seen the Kuffel Creek plans but they have an excellent suggestion to take the two center cross members of the HF trailer, replace them with a wooden cross member and install the steel pieces in the side rails. This keep the trailer from folding. I took the idea and went one step further, I stretched the frame to nine feet long by using two wooden cross members:
This was going to allow me to use two 2x6s as the end members of the floor frame. Each 2x6 would be attached directly to the steel frame and overhang about 3.5 inches giving me the correct floor frame length. I stretched the trailer toward the tongue. That is, I left the axle where it was supposed to be from the back and got a pretty good ratio. The black piece in the picture is length of 1.5 x 1.5 angle iron that give some additional support and makes the bottom of that stretched section level with the other members, important as the axle hanger goes across it.