I had typed a longer explanation for this, but then the lovely Macbook Air crashed and sent it off to unposted reply purgatory so I will keep it a little more to the point.
Went to the machine shop yesterday, kinda went the way I expected it. Getting much outside help for the work as described earlier is pretty much not going to happen. Not without considerable expense that certainly makes the project not worth it.
I have heard all the comments for cutting losses and starting from scratch, and may hit the reset button shortly on this. I did have one more epiphany this morning that I'm going to put in my pipe and puff on for a few days to see if it is all I think it is in my head.
Last try; M105 modded 2.0:
1) leave the floor intact, only cut the outsides to final dimensions
2) using my grinder (the only metal fab tool I feel confident in using), slowly remove the thinner sheet metal sides from the floor and wheel arches (weld bead on side-to-floor joints and arches are spot welded)
3) using the floor as my semi-suitable work surface, bring the sides in to the correct width and tack to the floor and front/back. narrow the front/back pieces and continue working to resize box on top of trailer floor.
4) after box is tacked and secured in correct size on top of floor, remove arches from floor. Move arches inward approx 3" on each side and recess arch cut in floor to match.
5) Cut outside edges of floor to final dimensions and attach box permanently.
6) Slide box forward on frame, reattach, and then deal with the axle stuff.
In my head, the reason this might work for me is that most of the work can be done with a grinder and a large pile of cutoff wheels. I don't have a garage yet, so scratch building a frame, etc would be really tough in the yard.
The great majority of welding is going to be sheet metal, which can be tackled with a 110V welder. That's really the only practical equipment I can get involved in at this point. If I have to scratch this and build my own trailer I can try to borrow shop space and welder and get a lot more help, but that will delay the project quite a while.
Any welding that is heavier or above my skill level, I can take to the shop to have completed. The cost of that will be minuscule compared to having a shop cut apart and reweld that floor.
Any advice on unforeseen obstacles that may not be passing thru the new trailer fantasy in my head are appreciated

.
Another change I've made to the original design is with the wheel arches. I like the overall appearance of the M105/3/101 trailers with (essentially) flush arches, but I'm still a little partial to the protruding arches on the older trailers.
In order to 1) allow max useable interior space, 2) save a little work on more floor cutting, and 3) make the trailer a little more visually pleasing (to me) and easier to spot the tires...I'm going to slide the sidewalls in while leaving the arches protruding like an M416. This looks to be a lot easier than it sounds as the arches are only spot welded with tabs to the sides. It also means that I only have to remove 3" of floor width on each side into the profile of the arches, as opposed to more than a foot on each side if I leave the arches as is.
Track width will be roughly the same as the Fj62.
For weight shedding, I'm going to remove probably half or more of the heavy channel supports on 9" centers under the box . Landing gear will be gone too for sure. Box will probably be roughly 1/3 original size in the end, and frame will have a few feet lopped off the rear. And that massive axle, brakes and suspension will be gone.
And yes, this was less than I had originally wrote
