Figuring that a top nut typically has less than 7mm of thread, you're playing it pretty tight. But since you only need another millimeter or two you have options. First put it together and find out exactly how it fits. Then consider dropping a spacer to get 1.5mm or so. Another option may be to face the head tube down a bit.
One way or another, I'm pretty sure this can be made to work.
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