You can remove everything and just have the bolt body. It's unneccessary though. All you need to do is insert the gauge under the ejector and holding the trigger in slide the bolt forward and close it ever so slowly. Any slight resisitance is as far as you need to go. I am not going to explain what all the stuff is with go, no go etc as there has been enough BS said on that matter. Checking headspace is a quick and easy job and all you need is the reference as to what gauge gauges what and apply it. Nothing with Lee enfields is rocket scienece, only some people here will argue against that point though.
The only reason I’m make this post is to give Para a hard time
Précis No SA/Rifles/1A (March 1960)
TESTS, ADJUSTMENTS AND GAUGING
(b) Cartridge Head Space: Gauges required: SM 139 - .064 in. SM 140 - .074 in.
Using the .064, place in chamber, remove extractor and spring from bolt head and insert into rifle. The bolt must not close. Replace the .064 with the .074. Test as before but the bolt must NOT close. (a)When a No.1 and No.2 head breech bolt is fitted, there must be a minimum gap of 0.050 from the underside of the bolt lever to contacting point on the body socket. (b)When a No.3 head breech bolt is fitted, the left edge of the resistance column of the bolt must not rotate beyond inner edge of resistance shoulder of the body.
OK bear with me here. I tried a SAAMI field gauge (.070) in a No. 4 rifle with a #1 bolt head and it closed with minimal pressure. Now if I understand it right, is it still possible for the headspace to be within safe limits because the mil spec gauge would be (.074)? Or should this arrangement not close at all. Would changing to a #2 bolt head help? Just want to make sure I understand all this.
Yes...it is not only possible, but very likely that the rifle is well within safe limits. British field reject spec is .074. Emergency (wartime) "drop dead" spec was .084, so you are probably fine, just need to used a milspec gauge.
Yankee Engineering makes a nice headspace gauge for the .303 Lee Enfield. Their gauges are based on the original British military specs in addition to being cleared for the extractor and firing pin. Here is a link to their web page.