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...
Slave cylinder for the clutch is on the driver's side.
I have an 04 6spd. and the interference is between the starter bulge on the back of the bellhousing, the pass. side spring shackle, and the firewall - at least with the MBRP 3.5" downpipe. It fits, but is within 1/4" or less of all three of these at the same time. Try the MagnaFlow 17927, it's what I'd try if/when doing it over.
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