Don't cut. Get some 3/8" and 1/2" Blair Rotabroach cutters and use them to cut out the spotwelds. Panel will come out easy; make sure you don't damage the trunk latch mount in the process.
Get the replacement panel, slide it in for a test fit and clamp it.
Use a level and jacks (nothing big, just enough to level out the car - better if you can find a level surface to start with) to get the car level. I found the package tray edge in the trunk to be an excellent location to check from. Once level, check the level of the taillight panel against it. This is provided the corners do not guide the panel for you. Check side-to-side alignment from the edges and at the keyhole and gas tank filler. Once you have it tested, install your end caps and check that the panel isn't too high (which will push the bottoms out).
Close the trunk and do a test-fit. Helps to stand about 30 feet from the back end of the car too - just because it seemed to measure correct (package tray could very well be off) doesn't mean it's sitting right either. Get the panel to where it looks squared off with the back end.
Once you like the location, screw it in with four or 5 sheet-metal screws to temporarily tack it in place once you're happy with the location.
After that, simply MIG weld it in place with a 0.25 wire (not flux core) and a 75% Argon - 25% CO2 gas mix. Find a buddy or get a ~$500 Hobart, Lincoln, or Miller MIG welder and have at it.
I didn't have much trouble as a first-timer, though my trunk corners were so rough that the first panel did not go in correct. Then again, I wasn't as careful with fit the first time. Follow the advice above, and you should be fine. I did, and it came out perfect this time:
Trust me: You'll need that welder again, so don't fret the cost.
FYI, it looks as if the bend on your panel missed the rear crossmember. Good thing.
-Kurt
