the newer production m14s rifles will generally accept a GI bolt without much messin around. The older norcs, like one i got yesterday, will generally need to be rebarrelled as the barrel sits deeper, protruding slightly into the receiver.
The norc rifle i got yesterday headspaces 1 thou over 7.62 nato go with it's norc bolt(

) and is the first norc rifle I have met with headspace under 4 thou over Go.... most are around 9thou.
If you want to tighten up headspace on the newer norcs, a excellent to new condition usgi bolt will or should give you a rifle in the .308 go headspace range ( after lapping the bolt in).... no good for surplus 7.62 nato. The bolt could be further lapped to 7.62 tolerances... but... generally the swap to usgi bolt is not necessary unless you WANT a .308 chamber. This where the TRW bolt is often chosen as it is longer by a touch than other usgi manufactured bolts. If you are after match or close to match headspace for 7.62 nato..... look for a used but serviceable HR or winchester bolt and you should end up fairly close with a bit of lapping.
Currently i have one bolt for sale in the EE, it is an unfired norc that headspaces 4 thou over 7.62 go.... in my rifle.....
My quest for parts is nearly done and i'm not anticipating getting anymore G.I. bolts. They are availlable stateside, but MUST have import/export certs to be brought in...... for one bolt... it's just not worth it..
Here is what i have done to procure these parts..... me and 3 friends sourced our parts in the U.S. and pooled the order together through an importer similar to Questar... splitting the import fees.... made it much more affordable
*** ALSO, a bit of a warning, the more you have to lap a bolt in, the deeper into it's "surface" hardening you go.... if you are lapping the bolt and removing more than 3 thou from the bolt lugs...... the newly lapped bolt may require case hardening before it can be safely used as lapping off that 3 thou has gotten you to the softer metal underneath and this bolt is now unsafe.