I would say that if the neck of the new chamber is too narrow to allow a .318" bullet to slide easily into the mouth of a fired case, then buy a resizing die and take your .318" bullets down to whatever groove diameter the barrel proves to have. If the .318" bullet does slide easily into a fired case, then it should be OK to use as is. .002" difference is bullet diameter makes no difference, it the cartridge case will release the bullet when fired. I have found that resizing jacketed bullets up to .003" per stage is entirely feasible, and has allowed me to reduce jacketed bullets up to .009" in three steps.
