Extensive internet research tells me that this is the firing order of the Ford Voodoo Flatplane 5.2 litre V8 found in the Shelby GT350 and GT350R.
Firing order shown here is 1 5 4 8 3 7 2 6, the only possible firing orders for this engine are:
15263748
15283746
15463728
15483726
17263548
17283546
17463528
17483526
This engine is unusual for a flatplane V8 as the big end journals are positioned "Up Down Up Down" rather than "Up Down Down Up" as in most flatplane V8 and inline 4 cylinder engines.
