My vehicle had no fuel tank when I got it,
being a failed project someone else started and gave up on.
I see cylindrical shims used, but guess you have some fettling to do, to make it fit.
That seems too be the Landy way, and many measurements seem to be less precise than the butter on my toast, like the use of shims to make the tub sit in the right place for example.
I suppose if that Tank fits a series 2, 2a, 3, and early defender; then there may have been some veriety of mounting hole placement over the years, plus I find non OEM panels always need a bit of fettling to fit anyway. Should the chassis be drilled, at least you already have the right paint to re-touch your finish.
Be interesting to see what the suppliers say