I was thinking about that step.  The end of my barrel mics at .642" dia.  I read somewhere online that the end of the 1903A3 barrels were .645" dia. where the sight fit on.  I'm keeping my fingers crossed that a 1903A3 sight may be a line to line fit or slightly loose.  In a couple weeks I'll get the parts and we'll see.

I almost forgot to mention, at first I was considering using the band front sight from a No1 MkIII Lee Enfield.  That definitely would've required turning a little off the diameter at the end of the barrel.