Chubby is 67" wide out to out and 120" nose to tail on a modified HF 4x8.
Modified might be a mild statment since I tossed the frame away and only use the tongue assembly and the axle cradle but I am familiar with the frame and its pecularities.
I would not try and cantaliver all 4 directions. Let the rear fall on the rear crossmember. If I were to do it I would make a 2x4 halo 5' by 10' or your desired dimensions with a 2x4 frame standing on edge.
Note : I had to go 5' 3" wide to clear the tires and then the 2x4 halo making the trailer 5' 7" wide for tire clearance but I only raised mine 2" your floor might clear but it is something to check !
Internal "joists" would be ran side to side shifting as needed to hit the cross members but do not excede 24" center to center except if necessary at the wheel wells.
Apply a diaphram to the bottom after checking diagionally to be certain you are square . I would probably use 1/2 " sheet goods. Next I would fill the cavities from the top with blue foam board this will insulate and add quite a bit of resistance to racking of the floor. Next apply the top diaphram this acts as a truss to help keep the 2x4 joist from bending. This is your floor. The H.F. tongue is rather anemic for a 1300 #+ 5 x10 trailer. My album shows 3 rebuilds to avoid problems. Leaving the stock tongue in place a 2x2x1/4 sq tube can be added to the center and I suggest it is ran from front to rear and bolted or welded to each and every crossmember. Extend the tongue 18" to 24" longer than the stock tongue for improved towing and backing up. The pictures in the album pretty much show all you need to know. Now install your floor but let it start flush with the rear crossmember remembering the now extended tongue and existing tongue can be utilized to support the leading edge of the trailer floor. Here is your base and a nicely reinforced tongue that will handle the weight and support the added length.
Here is Harvey's " Nobody 's " tongue repair
