Yeah, quite a few old Marlins have it. None of the cures is very cheap. Set the barrel back - well, not on a valuable one. It might be okay on a shooter, but for the cost, you could find a 336 without headspace. I have a 36 in 32Spl with an exceptionally sweet trigger - and headspace. I went to light cast bullet loads and neck sizing which took care of the symptoms. It still has headspace, of course, but it ain't getting any worse that I can tell. This is a price bargaining point if the seller touts it as a shooter and there's end play in the bolt. I've known people to build the locking bars up on Wrenchesters, but I don't know anyone who's done that on a Marlin. SW
Yes, the locking block on a Marlin can be built up, but it is best to get a new one rather than build it up. I've done it on old ones, where the part isn't available, but I wouldn't do it for any but my own. The metallurgy could be changed by welding, and since I shoot only very mild loads in mine, it's not a problem.