The barrel from that website looks like a Green Mountain barrel. Last summer I got an L1A1 kit from IO Inc and it included a Green Mountain barrel.
I have 2 of the Century receivers. Both were extremely tight in the barrel threads. I considered buying a tap, but read on FAL Files to use some lapping compound on the threads. It worked great, and was much cheaper than a tap. The L1A1 bolt hold open is bigger diameter and fit into the Century metric receiver.
At first the kit didn't have the barrel shipped with it. Then a week later I got a package from IO with a nice RPK barrel, called and they shipped me the right barrel and a shipping label. Not bad to work with, they fixed their screw-ups.
According to research, either gas block will work. Just remove the pin and tap off.
The rear sights have differant heights (I think the Inch is taller). So if you install a metric gas block you'll have to find a metric rear sight.
Thanks, the reason I ask is because Apex has SA barrels without the gas blocks for $60 as opposed to with the gas block for $120, so I plan to use the L1A1 gasblock on it, but I wanted to be sure it would fit first.
I also plan to go with an Entreprise receiver now if they aren't too much more expensive.