Your HF 4x8 didn't come with an axle already? Why not use that one?
You can certainly do a full width axle on 5 wide. TPCE is 64 inches out-to-out and my Dexter axle is 72 inches hub-to-hub. I like the fenders outside of the walls, too, but it does not match the track width of my Jeep, making the trailer much wider at the fenders. If you already have the HF axle those are good enough reasons to do the half fenders and have "the look" of extended fenders without the hassle and expense of changing everything. I guess it just comes down to your priorities... part time camper, speedy budget builder or serious boondocker, money and time are no objects... there's a full spectrum of samples here to compare to. I did the Jeep suspension on TPCE because I am going deep "out there" in the mountains on a sometimes rough forest service road, and metal fabrication is in my background. Don't underestimate the difference in cost, either. My suspension was an E-ticket item ($$$) on my frame build (wheels & tires, then brakes and suspension). Standard trailer stuff is much more thrifty.
If the front wall and bulkhead wall are located over xmbrs on the steel trailer frame, those walls will act like super rigid beams supporting the side walls. The side walls are super deep beams spanning between these two supports (assuming everything is glued and screwed well and remains water tight... no rot, and that you leave a few inches under the door intact so the wall maintains its integrity). The field of the floor will be supported by the trailer frame as well. That very short span of 6 inches between the wall and the trailer frame is less than the spaces between xmbrs in the field of the frame, so there should be no worry about sag. It's not like you will be standing or laying there. The walls hold the floor up, not the other way around. Make sure that you screw into solid cleats (not the edges of the ply) and you will be dandy!
The reason I have my walls sitting on top of the frame rails is to protect against bottoming out on rocks and stuff. The walls will stiffen the frame, not the other way around.
You can clad the lower front with some thin alum diamond plate to protect from rock chips and dings (I plan to do this, too).
If you feel you must make a subframe out of wood, think 1x2's on edge, or even 1x4's flat or on edge, not bigger (unless you are trying to space the floor above the fenders, in which case you should seriously be looking at under floor storage. Check out
Zach's XII. IMO his floor is still over built, but done very well. He does bomb down forest roads, and he builds convertibles (in engineering terms, no rigid roof membrane/diaphragm), so I guess he felt he needed the extra strength in the floor. For a hard top I would still consider 1x on edge instead of 2x.
These are just opinions that I have formed from reading a ton of info here and paying attention to what people who have done it have learned for themselves. I can't ever recall someone saying that they wished they had built heavier (except for maybe weak single tongue tube failures), but there are many, many examples of people saying that they coulda, shoulda built lighter, and will do so on "the next one".
I'm sorry if I am not helping you make any decisions. It is hard with all of the options and choices to make. Keep planning. You will build what your senses tell you is right for you (TLAR) and it will work out great!
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