Paul - your engine is mechanically timed by the way the cam and valve timing events interact with the position of the crankshaft. Unless the mechanical parts are physically changed to a different arrangement - no other "firing order" is possible.
Numbering schemes are a different matter - all sorts of numbering schemes can be created (where is number one this week?) - but the order in which the cylinders reach TDC on the compression stroke is mechanically bound.
There are differing cam timing schemes that can be tried - but it takes a new cam. The Y-block uses the same firing order / mechanical scheme used on the flathead. It was designed to reduce torsional effects on the crankshaft (flatheads had fewer main bearings) so the combined bending and torsional stress are (to a degree) minimized. Racing scrubs have been altered by building special cams - to use the flathead firing scheme.
Steve Metzger Tucson, Arizona