Good advice!If memory serves that rifle used something like five different bolt heads of different length to achieve the correct headspacing. Suspect you would need to measure the distance between breach face and bolt head then order the correct bolt head length required for headspace.. Likely would have too locate a source for those bolt heads--possibly Numrich Arms.
This is still .303Brit, right? Make your own headspace guage: cut a small piece of soda can into 3 disks the diameter of the case rim. Now try one stuck on a sized case - you can use grease to hold it in place while you attempt to close the bolt. Now try two at once, then three at once while you attempt to close the bolt. Each shim is approximately .004" thick, so you can tell "how far off" the headspace is by adding shims until the bolt is difficult to close. My No.4MkI will barely close on one shim.
Also, since you reload, you can custom-size your brass to alleviate headspace by partial sizing as follows:
1. Expand the necks of new brass to 8mm or so.
2. Back your sizing die out at least one full turn.
3. Size a case & try in your rifle.
4. When the case will chamber, screw die in an additional 1/8th turn and lock it down.
I notice you're using premium bullets - I'd recommend cast bullets to fire-form cases, or lighter loads using bulk bullets like Remingtons. Also if it fails the "three-shim-test" by easily closing - straight to a gunsmith!