Went back to reply #18 Quote " I was unable to fully close the bolt using only the lightest pressure.  However, I was able to close the bolt using a little more force when using a 30-40 new unfired brass casing."
If this is correct that you could close the bolt with very slight effort on the oversize 303 gauge your head space is correct. Something else is causing the case to be tight.
Here are some things I can think of, maybe others can come up with other reasons:
#1 Debris in the chamber that has resisted cleaning
#2 debris in the extractor slot stopping the bolt from going fully forward when a rim is in place. looks unlikely as the later photo shows very clean, check near the barrel threads just in case.
#3 chamber is not reamed deep enough on the shoulder    I would think this is unlikely on an worn original barrel. It could possibly be a combination of putting a tight new bolt with a shallow chamber.

If I read that #18 correctly and you can close on the oversize gauge we have been barking up the wrong tree, please clarify if I read it wrong.