I finally completed the install of the mid-crossmember. I first attempted Option 1 above, but I was not able to slide the crossmember across. Being an '84, my 110 frame is a little different from later ones, especially in that area. I also didn't have a friend to assist me.
I therefore had to go with Option 2. Initially, I took a piece of the old mid-crossmember and tried to pop it back in using this method, but without any jacking. At that time, I must have decided that it was not going to work, which is why I started with Option 1. I then used my floor jack and a piece of 2x8 vertically to jack up the floor support closest to the rear seat/mid-crossmember area. I was able to place the board on the lip of the floor support. Just like YRM said, I jacked up the floor ~1/2" and the crossmember popped in. It was so easy, I actually laughed. I also purchased YRM's SS hardware to complete the install. I then discovered I had to insert the bolts closest to the rear floor before the crossmember installation, as they won't fit after the crossmember is in (Some folks have used shorter hardware and install them upside down). These are the long bolts that vertically connect the crossmember to the frame. There are four. With the two bolts now in place, I was able to jack the floor up a little more and pop the crossmember with bolts into place. The remaining two bolts can just pop in after the crossmember is in place. I had to use five shims (per side) between the uprights and the tub. I bought PL ones from RN.
If I could go back, I would have started with Option 2. But I now know how to use a pneumatic rivet gun having replace the sail panel. I pushed too hard on the first one and the tip bounced off of the paintwork leaving a chip... right of passage stuff.
NOTE: I was OK with jacking up the floor 5-10mm as YRM instructed. Jacking the floor up a little more (to fit the bolts) I became uncomfortable with. I did it, everything bolted up fine and I'm happy with the result. If you have questions, just ask YRM, they were very helpful. I put 20lbs of torque on each bolt from a later Defender body torque settings list i found on the web. My manual does not list them. When I ask YRM what the torque figures are, they stated "On our personal vehicle fixings are bolted up tight."