I love my 404's!!!!!!!!! The best thing about them is to be able to air down to 4 PSI and still have more ground clearance than others, just about fully aired up. The NE is real slippery lots of days and it really helps with the airing way down.
Anyway, I had to move the diff over in the stock housing to get it to fit (4.25 inches for my full size chassis, which put it right where a full size 78, D60 would be). I currently have my 406's stuck under my 74 I'm building and I see no way of them fitting without tubing the front forward of the motor mounts, both outward and upward. You can get around this with lots of lift, but even a 3.5 (added to the true portal lift of 4.5) gets you way up there near 8 " of lift (not counting any body lift) .
Mog/9's are a great idea and can fix a lot of the problems, but you are looking at another 3 grand or so per axle by the time you get housing, new custom inners, ARB, gears, bling 3rd member case, and NASCAR big bearing pinion setup. It does fix the 404 mogs biggest problem (from an install standpoint) being the huge snout pinion it has. With a WB of 104 in my fullsize, I barely have a front driveshaft, I just don't see it, at anywhere near the stock WB of an EB.
The suspension is going to have to be BEEFY if you "get it" a lot. I have broken just about everything (links, link mounts, leaf springs, traction bar, track bar mounts, drag link, front axle truss, and recently a cracked frame where upper 3rd link mounts) that holds the mogs in the truck. By their nature, they just want to rip themselves out, even when the truck is sitting still. On my 74, I will be using EVO large 1-1/4 joints for all links and 2.25, 3/8 wall tube.
Some have broken portal boxes. Many think it is attributable to loose portal box cases. The bolts due loosen regulary and need to be tightened. I have not broken a box and have had my 6300 lb pig hopping on 42's many times (I do not have much HP though, so that helps and I do make sure my bolts are tight almsot every run). It does seem that recently, finding a spare box has been a bit more of chore than it used to be. They were going for around 300$ for a spare. I would not run without having a spare, at least back in the tow rig. Not like you going to the junkyard for parts if one does take dump.
I pad 2400$ for mine with pinion conversions already. Disk/8 lug conversions are going for over 1000$ per end these days. Current;y, I'm paying about 1300$ for custom inners on my 406's to move diff closer to the stock EB (ford) location (about 4 inches over). The 406's use large but std looking axle tubes where the 404's use double wall housing with a 3rd member that is rather hard to cut and fab.
I think the bottom line is how hard do you wheel and do you hit the diffs a lot now (and does it slow you down). Lou seems to be able to drag himself over a lot due to his very low weight, so the mogs would not probably help him much.
Not sure most could not get down to his weight (I'd guess) without making a narrow buggy like he has (which you may not want??).