ok, here goes ... and remember, this is probably what i would do, but doesnt mean it is your only option. i would not use anything under 2mm steel for the chassis, 1.5mm for the floor section, if you can get it and dont mind working with it use galvenised, but remove the coating from anywhere you are going to weld, because the fumes are poisonous.
1) decide if you want that little bulge in the chassis to accomodate the standard bump stop, this will just get the decision out of the way now, it wont make a huge amount of difference to the amount of work.
2) remove the exhaust it will only piss you off and get in the way, while your at it, get rid of anything else like the handbrake cable, LAV system etc etc so you have plenty of room
3) if you have 4 suitably large axle stands (and 2 smaller ones for the front) then get it up on them now .... carefully, if not then dont panic just get the back up evenly
4) now get in there properly, and have a look how far the rot extends, dont cut it all out yet, just get some markings to show how far it extends, pay particular attention to the area around the shock mountingand also towards the point where that chassis rail meets the outrigger rail, as there is usually a strengthening plate inside and this tends to rot out
5) you have already cut the floor, now make sure you have cut enough of it, clean it and surrounding area (all the way up to the edge of the inner wheel arch panel as that looks rusty too) remove anything dodgy.
6) take a cardboard template of the side section you are going to remove from the chassis (use the side you havent already cut out, making marks to indicate start and finish of template on the van
7) now cut out any parts of the chassis that is rotten using your marks from earlier as a guide, try not to lose the position of the shock mounting
8 ) now it is hard to tell from the picture if both sides of that section need to be cut out, it looks like you might get away without removing all of it, if this is the case then time to repair the floor, weld it in from the top, and then from underneath (inside the box section) and also spot weld it to the original piece of the floor directly over the original join area between the floor and chassis, this is because one half of you patch will not be directly welded to the chassis, if you have to remove both sides of the chassis rail, then just cut the floor 1" away from the join between chassis and floor, then repair the floor section
9) now repair/fabricate and replace any strengthening sections that should have been inside the box section
10) now use your template to create one (or two if needed) side section(s) to the box section, but make your panel 10 to 15mm deeper so it sticks down lower than the original sides, fold the top at 90 degrees as per the original, line them up and see how the template markings look in relation to the original chassis rail, now you want to get that original shape, so draw a line just above that line and cut some suitably placed slots from the bottom to that new line, then bend the lower section inwards (opposite to the upperbend, thus recreating the original chassis shape) drill some holes in the upper 'tab' that you bent earlier to allow you to spot weld it to the floor (do the same to the second section if you need 2)
11) now if you are replacing both sides of the box section (if not go to 13) position and spot weld the inner one (furthest s away from you) to the floor, and tack it to the original chassis section (butt weld the edges that meet the original chassis)
12) repair any inner strengthening ponits as required now you have one section replaced
13) fabricate a 'U' section that will fit inside the finished box section, about 4" long, add a coupe of ribs at the 2 90 degee bends, and drill a hole in the bottom and weld a suitable nut inside. This will be for the bump stop mounting (even if you dont plan to have it now, maybe change your mind later)
14) fit this strengthening piece inside the box section to your new panel (drill box section and spot weld from outside then seam weld it inside the box) when fitting it, cut away the lower 'tab(s)' so it sits flush with the lip on the bottom of the side section, and weld it to this also, grinding welds flat on loer section
15) now do the same on the other side of the box section that you need to repair, remembering to spot weld it to the floor and also the bump stop plate.
16) you should now have most of the box section replaced (but only spot welded, this is so you can remove anything easily if needed)
17) fabricate a lower panel, that fits the whole width of the box section. and drill a hole in it for the bumpstop bolt, also drill small holes along each edge and if you can create a 'step' at one end so that it will slot inside the original chassis rail, at this end, drill the chassis rail so you can spot weld it to your new section
18 ) start at the end that slots inside the original chassis rail and spot weld it in place, beating the living crap out of it where required with a big hammer
to get it to follow the correct line of your new side sections, spot welding it to the lower tags that you created. and also spot weld it to the bottom of the bumpstop bracket
19) all spotwelded in .... cool now seam weld the whole bloddy lot once you are happy of course, and dress as required with the grinder
20) last bit
.... drill 2 holes in the bottom of the new section, one at the lowest point each side of the highest point (ie the bumpstop) make them about 10mm, these you will use to fill the section full of waxoil and they will also act as drain holes, if you can spray some paint in there first that would help(if you used Galv, no worries, save the paint
)
thats it one nice strong repair that will last you a while, and without having to bend stupid shapes, and if you take your time making the side sectiosn and weld it correctly it will be almost invisible as a big repair, you could of course use a repair panel, but you are likely to spend almost as long screwing around trying to get it to fit correctly, and i doubt ti will be as strong
). I'd have though that that side being the shitty side of the road would have gone first.