"383 lift" could
mean lobe lift. I just love it when statements are made so clearly. No decimal points, no nothin'.
As far as "everybody I know says they (L31 Vortec heads) will support 480 lift stock"- do NOT count on it. There have been cases where they will allow 0.480"
lift, but that's going to be w/o much if any
leeway between the retainer and seal at max lift. There are easy and not so easy ways around this, but in stock form this is a total crap shoot. More on L31 Vortec heads here