I'm running a tubular upper arm for a 55-57 that's been shortened. The taller spindle and increased SAI requires a shorter arm - should be obvious. I have no idea what a C3 upper arm looks like or where its pivot is located.
I don't think anything is different in tire scrub between stock suspension and a suspension with the C3 spindle. 0.4" seems like a lot based on the geometry, I'll check my layouts.
Edit: my layout shows 0.17" scrub in the first inch of travel when you lower the car 2" with a shorter spring.