Here's what I did: Marked the new flex tubing around it's circumference in 6 evenly spaced places, 3/16" wide each.

It worked out great...I had to remove 1-1/8" of material..or 18/16". 6 cuts of 3/16" each...perfect.



Bent the pieces inward, to make a cone...

Inside view:


And finally...a nice venturi, just over 2" ID at the smallest point:


(Yeah, the welds look like shit...I had to set the heat too low to not burn through...stupid flux-core!)