Len Savage built me a 8mm Bren using the exact barrel conversion process you describe.. and an Inglis - Resistance Bren receiver... We ended up using a standard .303 bolt so that he had to machine the barrel allowing it to seat approximately .10 or so further into the receiver... it is my understanding that the true 8mm barrels were a bit longer and when mated with an 8mm bolt... made up dimensionally for the lack of "ears" on the barrel face etc. I think the 8mm/7.62 bolts are longer than a .303 as well. A second is still in production... but it will use a true 8mm barrel and 8mm bolt... and should not require any machining to make up for dimensional differences between the .303 and 8mm geometry.

Per Joe's experience with a .303 receiver... I too have one of the "unknown maker" 8mm converted .303 barrels that swaps out perfectly with a .303. The same gun also performs flawlessly with a south African 7.62 barrel and the original .303 barrel... all using a standard .303 bolt... (but I have two so that I don't have to swap extractors).

I'm sure our armorers can expand on the various dimensional differences between the .303 and 8mm Brens... such that you can properly set up your converted .303 barrels.