Seems like the right size, fit it in there and drove it today to seat the piston rings and speedo is a no go. Are the AOD and C6 speedo gears compatable to each other?
AOD and C4 speedo gears are the same, or both fit either. Not sure about C6's, but if you put them side by side, and the teeth are cut the same way and they fit, they should work. To be sure, you could ask a dealer parts guy to cross check the numbers.