Ok, I've got the sizing die figured out now. I had to screw the decapper/expander all the way down until it could go no further. I then "sooted" a case neck to determine how far down to screw the die body. Just before contacting the shell plate, the sizing line was just a hair above the shoulder. Any further down and the die body would contact the shell holder before the full stroke was complete.
So I think I've got this problem licked.
Now on to problem number two. Which powder funnel to use in the XL650 powder measure!! I thought I had one that would work, but because the case is so short, the powder bar doesn't travel the full distance. So now I need to figure out which powder funnel I need.....
ETA: According to Dillon, I need the 7.62x39 powder funnel. So I ordered up one of those. I think this project is going to "nickel and dime" me to death.... But it should be worth it in the end...