I've done many cars. Mostly Mustangs and I did the trunk in my brother's '71 Goat and another '69 Chevelle 396 (Nice!).
From what you've written I guess you are doing the entire trunk. The hardest part will be the seam. You have to decide (with the help or your friend who will be doing the welding) what your goals are BEFORE you start anything. You can either try to make it look like it was never changed (very hard to do and requires hours of tedious seam welding/cooling/welding grinding/sanding/sanding more) or you can decide to just have a nice sealed lip all the way around the patch panel that can be spotted by either the trained eye (if you really take your time, use the flange method, and do it right) or anybody (if you do a fast job that simply works with the overlap method).
-First, get a patch panel that is bigger than what you are going to cut out. -Cut out the area however you need to. -Get a compressed air flange tool like this one . -Flange the PATCH panel all around so that it fits into your cutout pretty well. You'll need to cut metal off the patch panel in order to be able to flange it so that it fits in. -Use the Flange tool to punch holes IN THE PATCH PANEL that will be used to mig weld it to the floor board. I haven't done a car lately, but I think I used to do a weld every 1-3 inches depending on the strength I felt I needed?. -I always like to sand and prep the areas and paint with a weldable primer. -I always used self tapping screws to screw my patch panel in first before welding as a last chance sanity check. Look underneath and see that you have made a pretty small gap between the trunk and inner part of the emboss on the patch panel that will need to be filled with a sandable seam sealer or fiberglass based filler if you can't find the better seam sealer product. -Weld that puppy in there using the mig welder! Start in the center of the hole you cutout and work your way out in a circular pattern making sure to really weld the perimeter of your hole with the nice solid weld you started in the center. -I always like to grind my welds flat but this is just a personal preference. -Finish it off by sealing everything. Sanding. And priming.
I've pretty much described what to do at the perimeter of the part because it is the most important from a cosmetic point of view. You will have to weld the panel in all the same places the original panel was (you'll learn this when drilling out the spot welds of the original panel. For these interior portions of the panel I usually weld from the underside of the car through the hole that was created when I drilled out the spot welds from the top side.
