What are the recommended steps to deleting the side marker lights?
Will I need to remove the gas tanks, to properly rust-proof the back of the blanks?

 

just remove the tail lamp lens, then remove and disconnect the tail lamp housing,
and you can see the rear side marker mountings.

 

I did this and other Euro look details to my coupe. The side markers are held in with two screws that perforate the body. These holes are about 1/4 of an inch. There is an additional hole for the lamp socket and wires to pass through. That hole is about 1 inch or so. The screw holes can be plug welded up using a copper spoon on the back. The rear most hole is near a body crease that will warp like crazy if you heat it up. Guess how I know...on the other side, a taped up a wet rag in place for cooling which
reduced the warping significantly. The larger hole needs to be patched with a piece of metal which is easy to cut out but on the rear there is a slight curve the metal which should be accounted for before cutting the patch. Lay a flat piece over the area and use a large diameter pipe or english wheel to set the curve. Then trace the hole from the back side and cut it out with a tin snips and weld it up. Metal finish and body fill if necessary.

The front is the same procedure but the fender should come off and the area is flatter so no curve needs to be set.

The repair needs to to be heavily seam sealed and undercoated on the back side to prevent a moisture defect.

An issue comes up when you no longer have side markers. Technically thats a violation and somewhat un-safe as a practical matter. With the Jags recessed head lamps the car is virtually invisible at nite from the side. Dangerous when you're crossing the Taconic (local joke) or any other road. To solve this I wired up 1997 Xj6 side blinker in the fender to come on when the parking lamps used to, and to a relay interrupts the ground so it blinks with the turning signals. You can see it in the pictures in the posted linked below.

https://www.jaguarforums.com/forum/x...1/#post1002172

 

I'm thinking about fastening some LED lights below the upper flare of the four fender openings. This is similar to the Mini Coopers but much slimmer. These can be brighter than the stock markers.