It may have something to do with the clutch linkage. My 4.3 Blazer has the slave cylinder on the passenger side. 4 inch is a pretty big down tube. My friend has that set up on his 03 F350, not a whole lot of difference between his and my 3.5/4 inch set up as far as sound and performance goes.
Maybe you can find a 5" that has a 3.5" down tube...
Maybe you can find a 5" that has a 3.5" down tube...