12 bolt spider gears binding, help please!

[Reyals Bemus]

Long story short, I need to replace the spider gears in my open diff '64 12 bolt because I am converting from 17 spline to 30 spline axles. I bought a kit made by Superior that includes both side gears, spider gears, and thrust washers to go behind each gear. However, when I have it all installed in the carrier, the gears are virtually binding against each other. If I stick an axle in and try to turn the diff, it takes some effort and the gears are very notchy because there is zero play between them. Its like the thrust washers are too thick, especially the conical ones behind the spider gears. Without them in place however, the gears seem too loose.

I should note that the original spider gears did NOT have thrust washers behind them, but the side gears did.

Now I'm pretty confident this 12 bolt hadn't been abused. The whole truck is in good shape, and everything looked really good when I opened up the diff. The main ring & pinion backlash is perfect, the teeth on everything looked perfect, etc etc. So I don't think the carrier is warped or anything like that.

Anybody have any ideas about this? How are the spider gears supposed to feel? What kind of play is acceptable?