440 breaking up at 5600 RPM

Check your entire fuel line for kinks etc.... You are running 3/8 right ?

I went through a near identical experience with a W2 .638 roller motor... Changed entire ignitiin system, rejetted many times,
Never could figure it out.
Then one day I was changing gears and I looked up at the fuel line coming out of the tank....it was kinked ! Fabbed a new section of line and Voila .... problem solved...