The time estimate sounds about right depending on whether you have to deal with any rusted bolts, etc. If you're replacing the head gasket I would go ahead and do a full valve job on the head and have the valve guides checked too. I would also put in a new timing chain & guide.
I'm not sure if the heads are interchangeable or not, I suspect they are but I don't know.
Randy Rosetta KDØKWX TLCA #19351 69 FJ40 "Meanie" & 1970 Boss 302 Mustang