A leakdown tests the entire front crank/cylinder cavity. Will check for leaks at crank seals, case mating surface, base gasket, reed/intake areas, and head and spark plug seal as well(I have had a spark plug I thought was tight leak on a leakdown before). If it is a cool head it could be tough to find because the leak may just be into the inner cavity of the cool head and sealed perfectly fine to the outside. But if it is not holding the 6 lbs and you can't find it leaking anywhere else, most likely that is your problem.
On the pipe subject, I think it is the head gasket, but you are going to have the pipe off anyway to plug your exhaust ports. See if anything dumps out of it. Even if it was a crank seal, and you fixed it and never dumped the pipe out, it is gonna still smoke till it burns most of that oil out of there...