2007 Lucerne CXL. and 1995 Lesabre currently, past 1973 Riviera, 1968 Riviera
Locate the pump sending unit, measure the distance in the trunk area, mark a area that will be big enough to remove pump, use a dremel type cutting tool, surface cut avoiding cutting through to pump, wires or tank, make patch panel, silicone the area and attach panel with proper secure materials.
All of these are pretty much the same process, but if you are looking for a exact instructions hopefully they may show up.
I haven't done it on this model car, but I have done it on several trucks, SUV's, etc. Usually due to serious rusting, or other issues that make removing the tank or the bed far to hazardous or hard.
My 8.1L suburban, with twin tanks and twin pumps I had to do it as they install the tanks before the body. Even removing the strapping, which was easy, and freeing all the lines I could from the body, I couldn't get it low enough to access the fuel fittings at the top. Add in the main tank is 30 gallons and the secondary is 10 gallons, and they are plumbed to each other like corvette tanks with both gravity feed and cross pumping to keep the weight equal, it was too much.
So with it hanging as low as I could get it, I used .250" Kydex and heat formed it to the interior floor panels first. Kydex is gas/oil safe and I made the panel about 3 inches to big in every dimension. I then cut the floor with a short blade airsaw and death wheel on my grinder, and saved the panel. I painted the metals edges with a quality metal primer and paint. I did all my fuel pump work, then riveted the cut out panel to the kydex and sealed the rivets with RTV. I used gas oil safe 1/4" thick foam to seal around the kydex and the floor and used rivnuts to put 6 (rear) and 10(front) short stainless bolts into the floor so the panel is removable whenever. I then put the tanks back in position. I can change each fuel pump in under an hour now. If I could have formed a steel panel it would be even cooler, but the floor is quite interestingly shaped in the front and ridged with stiffening rails in the rear.
The kydex is strong enough to deal with the blade width cut and the metal backing it up keeps it strong over the large span. as it's under the hatch carpet and carpet pad in the rear center, rivenutted to 2 structural rails, and under the driver side rear seat in the front. I haven't had any loading issues with it.