FWIW, I tried the S&S headers with my T56 and LT1 and could NOT get them to fit. The collector was pointing directly into the slave of the clutch, thus my adventure into homemade header making. I was also quite concerned about the passenger side ground clearance problem with the S&S units. Now they fit nice and tight with lots of ground clearance, although putting them in is a PITA, requiring them to put in from underneath, and lifting the engine enough to drop the engine mounts (and JTR spacers and setback plates), but since they will hopefully only be installed once (upon final assembly), I'm not too concerned about that.
Mudge, if your HP and TQ numbers are reliable, that puts a big on my face!! I'm sure that the Tri-Y type header setup that I have should give me a little more too
Tim