I think the pinned and welded fix is for making a barrel length 16", not an overall length fix. I believe an 18" Tavor with the proper length muzzle device is good to go as far as OAL without pinning or welding the muzzle device on, I could be wrong though. Either way my 40th birthday present to myself will be an 18" Tavor in Black unless they offer an OD green model!
Unfortunately at 27 5/8" the 18-inch comes up a little short unpinned too: http://iwi.us/TAVOR/TAVOR-TSB18