Paint and Body Level Car To Replace Rear Tail Light Panel?

I decided to redo the fuel system in my car. Tank is old and rusty inside, tired of the carb gunking up. . .I really didn't want to start a project like this, but since I have the tank out I think it would be prudent to repair the ass end of the car.

The drivers side of the floor is in great condition. The passenger side is getting thin with a few pin holes showing in the last 6-8". The rear trunk floor is paper thin and cracking. The tail light panel is thick with bondo.

I have the repair pieces needed. I need to know how critical it is to get the rear of the car level. This will be done in the driveway, with the driveline installed (unless otherwise advised). Is the body stiff enough to just do it as it sits while I cut the rear panels out? Should I break out the level and plumbob to determine baseline 0'-0" and jack the rear up and shim under the jack stands? This is a driver down for repair, hope to be back on the road after this fix is sealed and primed. Any and all advice is welcome!

Thanks!

Okey dokey. Since it will be on jack stands, might as well pull the rear axle and springs. . .

Where do you place the level? I was thinking the front of the gas tank mounting flange for leveling the rear side to side. . .any suggestions for the front?

Thanks!

That was interesting. . .quick check with the level on the front, the bubble was within the lines but slightly off. Checking the rear was hard since it is pretty lumpy (not sure WTH happened to this car in the past). I stuck a rod across a couple points in the rear and they all were within level. Not many equally smooth spots near the frame rails. . .

This was with the car sitting on its suspension. I tried jacking it up on stands and I mucked it up worse, so. . .

So uh, everything is out! That is definitely a big hole in the back I made. . .quick dry fit of the new panel nearly drops in, which is nice. If the rain quits soon, going to cut out about 10" of the rear of the passenger floor and the drop off.

Given your experience, would it be better to fit and install the tail light panel before the rear trunk floor goes in, or should I fit and install the rear trunk floor first? And, the trunk corner pieces, while solid other than that layer of rusty powder (that everything has inside), they have some rust bubbling out of the seam in the trunk channel. Still flat at the laps, so I'm thinking no cancer but do you think it can be treated and resealed? Or would it be worth the trouble to replace both sides with the pieces that came with the tail light panel? I was thinking I could clean it out with a dental pick and dribble some Ospho into it. . .

Thanks!

I went ahead and removed the trunk corner pieces. They looked fine on the outside but I could see the flaky rust in the trunk. Once removed they were pretty thin. . .the pieces under the end caps look great inside and out and I'm not going to sweat them. The ends of the quarter panels sprung in a bit basically closing the hood gap. . .this will be interesting prying this all back out to get it too look good :/

Hopefully I can post some victory photos this weekend!