Yup gotta take the engine out. Its cathartic. In my hayday, working in a shop environment, we could get an engine out in 45 minutes, replace the head gasket in 2-3 hours depending on how much cleanup and misc the customer wanted, and back in and running in about 1.25 hours. Those days are long gone.
I remember 3 of us swapping an engine completely in the pits at a drag race in less than 45 minutes. Of course there were a lot of street legal parts missing.
If its your first one, and you are really committed, I'd say out in 3 hours, headgasket in 3 to 5 hours, and back in and running in 3 hours. Assuming you've got all your parts in order, tools etc and nothing major goes wrong. Strip an exhaust flange bolt, or a cam tower bolt, etc.
Strict attention to detail should fix your leak.(s)
If you want to guarantee a no-leak motor, you'll need to take the cylinders off and replace the cylinder studs with heavy duty ones. Add 5 hours assuming you don't break one of the old studs getting it out. About a 50/50 chance.