I just had the same problem with my 74CR 400,while lifting the engine had to flog the barrel studs,soak in penetrine,flog,prise,vacuum,soak,flog for about three days.Without getting any of it down in the bottom end.
Haven't had the barrel off for about ten years.
The trick which I have used for years on my other bikes is to install O rings on your studs before you put the head back on.
Depending on how much room you have sometimes I use two per stud to get a good seal(the head should squash them down)
Clean the studs up and give them a good coat of Neverseize.