I am assembling a Sten 3 from a parts kit and need the headspacing to set the rear barrel bushing, the one from the kit was messed up and is of no use. I have new barrel bushing and barrel. I can find no information on the internet. The barrel is fixed in place once installed there is no adjustment
For the standard disclaimer. this is being build under the proper licenses and NFA rules.

I built some back in the 80s before new private registration was banned. Now only for Class3 dealers and manufacturers.
Since they are slam fire, the notion of headspace is of less importance; set the barrel so the bolt just clears the barrel face with the go gauge in place. Or a new piece of brass.
And on these, the bolt smacks against the barrel face anyway as I recall, so it's all clearance.