It's a ground joint, the spigot on the barrel should seat on the recess in the head for the gas-seal with a very small clearance between the outer joint faces to make sure that it's the spigot that's doing the work.
The practice I follow for this is to finish lapping in the barrel and head with fine grinding paste on the spigot and coarse on the outer faces.
There is at least one fairly recent thread on the forum relating to this.
Ariel torque figures (for bolts and studs) were not commonly given, for the singles doing the head bolts 'tight' is usually adequate. I think the best figure anyone has arrived at is 300inch/lbs(25ft/lbs).
There has been some discussion on this as well - the level of official documentation can be seen in this thread!viewtopic.php?f=7&t=6189&p=40462
(The doc referred to has all the figures for all the model range - don't expect too much!!)
Hope that helps
'55 Huntmaster, '56 VH, ' 51 VH, '80 R100RT, '00 Sprint ST (but all those Ariel parts can only make one running bike...)