I just changed out my expansion tank and I have a few observations, as I did it differently than instructed to here and I could not have done it any other way, I think. I took out the old expansion tank with the assembly it attaches to together. This was easy as pie. No hammering or pushing or anything. Though You do then have to unclip and disconnect the bottom hose as well ( don't know the name).
Then, I saw where the hard part was. With the old tank out, I ubclipped the bottom of the tank from the assembly and I could then slowly pry out the old assembly with a bit of force and a sturdy screwdriver. Viola. It would have been almost impossible to do this separation while the assembly was in the car.
Then I had to mate the new tank with the old assembly. Mere pushing with body weight is not enough. I was stumped into how I was going to mate them, as they were not wanting to. I reexamined the pieces to see if perhaps they didn't fit or something.
Then I got an idea that made it simple. I was able to mate the two by laying everything sideways on the ground and using my hydraulic car jack, also on its side, to slowly force the assembly into the new tank. No banging or grunting.
I did see how tough it was to mate the two and again, I can not even imagine trying this with the assembly in the car. Besides this unmating/mating issue, the next hardest thing is unclipping and clipping the lowest and 3rd hose as there is very little room, but not altogether that hard.
So if you can remove the assembly with the tank and then deal with the assembly once outside of the car, I think it will be much much easier. Also try to use some tool to squeeze the assembly together again. At first I tried placing everything in a door jamb, and that almost worked but the door jamb was a little too wide.
My assembly just unhooked from the radiator mountings and looked slightly damaged. I think whoever replaced the tank previously may have done this damage, making it easier to remove the assembly, but I am not certain.