I dunno what Old Scout charges or if he's still in business, but you can rent a .400" neck reamer from GBO sponsor 4d for less than $30 including shipping, there's a bunch here that have done their own, just an option for ya.
I do know Wayne York would probably do it for ya, he's done a lot of work for members here including me, but he's kinda busy tho, it will take 6-8 weeks more than likely.
http://home.earthlink.net/~oregunsmithing/Well, they didn't just fix the chamber, the SAAMI specs were from the original 38-55 that used a hollow based bullet that would expand the skirt to work in the larger bore. A Marlin Tech got wind of the problem with the 38-55 Target at about the same time that the .444 was being planned for reintroduction with the original 1:38" twist bore. This all happened when Marlin started making the barrels for H&R in early 2007, so he got the bore and chamber problems of the 38-55 cured as well as the twist rate of the reintroduced .444 changed to 1:20" which is the same as other Marlin barrels. And all was happy concerning the 38-55, but marketing apparently decided the 38-55 wasn't a good selling model, so they dumped it.
Tim