I don't think it was a machining issue with just my chassis... I think they just made them that way. It's designed to accept the older style steering mechanism, not the 3905's version.
No big deal, just had to buy something else to get it to work.
but they sell one for the 3905 and the 3906. sounds like you got the 3906 extended chassis. call up flm and they should give you the proper replacement.