Will taking a "pre ban" upper and having the flash suppressor welded, permanently attached, and having the bayonett lug cut off make it a legal "post ban" upper?   I have a post ban lower, but want a skinny barrel, normal flash suppressor looking gun, and all the ones i can find are pre ban uppers.  I dont mind cutting off the lug and having the suppressor perm. attached, just want to make sure it is OK.

You can't have a flash supressor on a post ban gun period.  You can remove the flash supressor and have a muzzle break installed.  If you cut off the lug then everything should be okay then.  I'm sure smarter people here will speak up about something I've missed.
Bill3508

YES, it will work but only if you use a muzzle brake and not a flash hider or you could cut off the threading with a hack saw and recrown it, Brownells has a tool to recrown the rifle it is easy to do. The loss of an inch wont affect the accuracy and it will help lighten your rifle.

Damn, what i wanted was the look of the birdcage style flash suppresor.  So what is the technical difference between a suppressor and muzzle break.  Is there a muzzle break that looks like the birdcage?

GJ,
 Go to Kurt Kustom Firearms website and look at their A2 and A3 TacBrake, it looks very similar to a flash supressor.  A flash supressor does just what it says it prevents muzzle flash, and hides your signature(shooting position).  A muzzle brake reduces recoil, but does nothing to reduce flash.
Bill3508