I've been working on a project which has been stalled by a headset stack height issue. The problem is that the Dura Ace 7400 headset I very much would like to use has a taller stack height than I expected. The headset does barely fit, but there are only perhaps 2mm (a couple threads) showing before I screw on the top locknut. The original fork is an aluminum Tange Fusion fork. I'd prefer to stick with the existing fork (mainly just to keep everything original), though I don't know if I'd have any luck finding a matching fork with a longer steerer tube.

So, I was in the LBS asking their opinion, and it was suggested that one possible solution would be to give the headtube a heavy facing--just enough to expose a few more threads for the headset. How much exposed threading is sufficient? (I'm guessing I will only need another 2-3mm, plus the 2mm or so I already have.) This seems like it may be the cheapest option and also allow me to use both the DA 7400 headset and the original fork.

What do you think? Please stop me if this is a bad idea!