On mine, I was able to just trim the bolt threads flush with the nylon lock nuts and it clears, barely. If you switch to a thinner, metal lock nut (i.e. the type that gets a bit tighter toward the end to keep it tight, like a stock shackle nut) you will gain a bit more clearance
I will take some pictures of mine and post up. IIRC, one side does fit a bit closer then the other. Instead of drilling the frame, I would just drill out the holes in the skid plate slightly larger so you could slide it a bit side-to-side, if needed
It is tight, but it will clear
The Kilby looks like it uses thinner metal for the side pieces, allowing a bit more clearance. It would be interesting to know if the two different skids use different gauge metal for the side pieces.
The factory, thin metal 20 gallon skids fit fine. But, from the factory, the 20 gallon tank equipped CJ's have the rear shackle bolts reversed, i.e. head of the bolt toward the tank, nuts out. The greasable shackle bolts that come with the RE lift wont allow fitment this way, unless maybe if you drilled out the other end of the bolts, sealed the hole in the head of the bolt, and basically reverse the zerk orientation on the shackle bolt

