Danny, I built my 72 inch "Stacker" on a 40inch by 48 inch HF Frame. I guess I am one of the fewbuilders that feel that a trailer frame is bolted to the super-unflexable trailer body simply to provide a connection for the suspension and the tongue. My Stacker body is a ridgid all-wood unit glued and screwed together and weighs only 140 lbs. and with the 95 lb HF trailer bolted to the bottom, totals 245 lbs. If you check the pics in my Gallery, you can see my unorthadox method of gittin-er-done.
My 8 ft 1/2 Nelson Woody has a 6 ft HF under it, and the floor framing extends farther forward than it does in the rear, so the wheels are farther back to provide better handling and a tongue weight that is within limits. If you check out my Woody build pics, that can be clicked on below, you can see how I managed to conceal the entire trailer frame from view. If you have any questions, please PM me and I'll go into more detail. Good luck on your build, I'm sure it will serve it's purpose well.

Roly (PS, I just finished my 3rd build, the 3' by 3' TD, whiich provides me with a 75 inch bed and tips the scale at 85 lbs before installing the single wheel and hitch, with absolutely no trailer frame below it.)