Forget about the specific parts for a moment and consider what's needed.
With steel headsets it's OK to press by the rim of the cup, since the part is plenty strong enough. However with an aluminum cup, is important to press from the center so the cup itself isn't stressed. The "cone" side is easy since you'll always b pressing from center.
If you don't have a proper stepped sleeve, you might try dropping the cone into the cup (without balls) and seeinf if it's tall enough to protect the rim. Otherwise feel free to improvise any adaptor which will achieve the purpose.
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