Thanks for all the feedback and encouragement. It's really appreciated, especially when things are going wrong.
John, I hadn't even thought of using old chassis rail as cross-members. That's a great idea. I'll check the local scrap merchant and see what they have laying around.
Charlie, I had never heard of a torque multiplier until you mentioned it. A bit of a google around and it looks like a very worthwhile investment. I went into town and asked around, but the one place that had one in stock wanted $2,200 for a Norbar one, so that was out. Both of the other places that supply tools wanted between $350 and $550 for the ones that they can order in. I came home and checked eBay and found a seller listing the exact same one that the locals were selling for $550. He has a Buy Now of $76 and $33 freight. It's a Miller Falls brand, so pretty low-end, but the others listed for lower prices were all no-name, Chinese made units, with wildly different torque ratings, although they all appear to be the same item. The seller with the Miller Falls one is away until the 14/1/2016, but that's ok as I have plenty of other stuff that I can do. Here's a link to the listing, if anyone is interested.
http://www.ebay.com.au/itm/TORQUE-MULTIPLIER-TRUCK-WHEEL-NUT-CRACKER-POWER-WRENCH-TOOL-TYRE-TORQUE-TORQ-KIT-/261880965593?hash=item3cf95269d9:g:tH4AAOSwPcVVnm9zTimo, I don't think we have any brake places around here that would do that. I got the contact details of a supplier of silica sand and other types of blasting grit just before Xmas but they shut down until after New Year, so I couldn't get pricing or other details. I'll give them a call early next week and see what they have and how much it's going to cost.
Now the good news. The damn thing is out! It took most of the day, but nothing broke and I only cut myself once with the grinder.
I wiggled and jiggled it around, working out what gave me the most room, to try dropping one corner of a winch cross-member down past the edge of the chassis rail. No matter what I tried, I needed another 5mm or so to get it past. While I was underneath, levering things around. I got a good close-up pic of the winch rope. I think it's beyond recovery.

The second pic is the rusted out upside-down cross-member. Most of that crap in there is rust chunks from the tray. There were 2 bolts coming through the bottom, stopping the spring hanger from twisting, but the nuts have totally rusted away. I'm not sure what is holding the bolts in place.
To spread the chassis rails enough to get the winch cross-member through, I used a scissor jack and a length of wood. It was a good idea, but not quite good enough. I was getting a decent load on it, but every time I got close to spreading the rails enough, it would flex a bit and shoot the bit of timber out sideways.

The jack also felt like it was very close to stripping the thread so I replaced it with the little hydraulic jack out of my Discovery. As long as I kept the pump tower on the bottom of the jack, it worked fine. I jacked it up again and could hear things creaking and complaining, yet still couldn't get enough room. I realised then that the cross-members in front and behind the winch assembly were too close together and stopping the chassis rails moving apart. I had already decided I was going to pull the front cross-member out and clean it and the chassis rails up, so out with the grinder and off with the rivet heads. Then I used a grinding wheel to take the remaining metal down enough to see where the rivet stem went through the rail. A couple of hits with the big punch and 4lb hammer and they shot out, leaving nice clean holes. Seeing as the rear cross-member was totally rusted out and had to be replaced anyway, I used the grinder to cut the back and bottom. The front was already rusted through. As I cut the last bit of metal, it sprang open about 5mm and the winch frame moved. I gave it a poke or 2 and it had started to drop through, but that one last mounting bracket on the other side that I couldn't get at was hanging up on the top edge of the chassis rail. At least now I could get at it. I couldn't turn the nut, as it was tucked in between the 2 gussets on the bracket, so I had to turn the bolt head. Luckily, while in town I picked up another 1-5/16" socket, an impact one this time. I was able to get one socket on the nut with a 1/2" to 3/4" adaptor and my spare breaker bar and lock it against the winch body. I used my new impact socket on the 3/4" breaker bar from work on the bolt and the damn thing unscrewed with no effort at all. It was the easiest of the 4.
With this bolt undone, I was able to get the bracket out and the cross-member lifted straight up, allowing the one diagonally opposite to drop down. Then it was just a matter of levering it all around until it fell through and my overhead winch thing took the strain. I got my ATV lift in position and managed to jack it up and rock the winch around until it was positioned with one support under the edge of the winch gearbox and the other under the wire rope, with a timber packer to keep it level.

Tomorrow I will lay out my sheets of steel and try dragging it around and down to the house so I can dismantle it. I can't believe how heavy it is. It's far heavier than the transfer case and I don't think my workbench will hold it, even if I could get it up there. I'll have to pull it apart on the ground. Maybe it will be light enough once I get the cable drum off.
Now that it's out, it's not a high priority. I want to get the front cross-member out so I can clean it up and use it as a template to replace the rusted out rear one. With those out, I can clean and paint the chassis rails right back to the rear diff. When that's done, I can refit the air reservoir and most of the air and fuel lines, as well as the transfer case. I'll have to make a new transfer case cross-member first.
Something I noticed when I popped the rivets out was that the holes through the chassis rail were slightly larger than the holes in the cross-member. I suppose the idea is that it will expand into the larger hole when it's fitted, locking everything tight. I'll measure the holes and drill the chassis rails out to the same size and refit them with the highest tensile strength bolts that will fit. I won't need to spread the chassis rails when I put the winch back, as I will be able to remove the cross-members from the winch assembly. Once it was down on the ATV lift, I gave 5 of the 6 bolts that hold the cross-members to the winch body a squirt with a new penetrent/rust breaker stuff I picked up today. Something I saw while squirting the thread ends is that all of the holes through the aluminium seem to have helicoils, or some other version of steel threads, same as in the top of the transfer case. Hopefully, that will make it easier to get them out.
Ah well, I still have a few days left to get things done. I'll post if anything interesting turns up.
Greg.