fuel gasket
			 
			 
			
		
		
		
		Not sure if this will work but it may be worth looking into.  I too have a 73 Mach I fortunatey my gasket is in good shape.  But I recently built a fox body (87) mustang and had to replace the filler pipe/tank gasket and it appears to be quite similar to the one on the 73 (only it comes in on the side of the tank).  You may want  to see if the diameter is close.  If so you may be able to modify it to work. 
 
Paul 
		
	
		
		
		
		
		
		
		
		
		
	
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