Hi Mark, copper works well, also coating them with the copper head gasket spray available at most auto parts stores. STeve Rinker carries copper ones stateside, maybe randy walker too. 

If you have diligently tightened the head bolts, ( when running in I carry a socket head wrench in my pocket and nip them at 1 minutes, 1 mile, 5 miles, 20 miles, 20 miles 20 miles until they stop moving), did you make sure that the gasket was not overhanging the combustion chamber ? There was a batch a few years back that a lot of people had burn thru, and it was mostly because they just had a small overhang into the cylinders Middle rear front I recall, and if you trimmed the edge of the gasket back a little so that it was sandwiched, they worked fine.

Copper head gaskets usually only need a couple of nip ups, whereas the composite ones often still snug up at 80 - 100 miles. 

I have been fortunate enough to own and used repeatedly, a couple of ancient incredible battered original composite head gaskets, probably a lot of asbestos in them but they are layered with copper sheet on both sides. The important part is that the edge against the combustion chamber is covered with copper and I think that is the reason the gaskets do not burn through, and of course nothing of the gaskets are overhanging the combustion chamber. I don't need and wouldn't use any sealant because of the risk of ripping them apart when unmount and cleaning process I have to do when reusing. 

I saw that recently, Oldtimer Service in Poland http://www.oldtimerservice.net.pl/engine_transmission.htm is stocking a composite gasket layered on both sides with copper. Composite gaskets does allow a bit more movement between the head and cylinder compared to solid gaskets, when the parts contract and expand with heat and that is very advantageously especially with aluminium heads on iron cylinders. 

Solid copper gaskets needs a bit more fettling to ensure a tight seal. The cylinder and head surfaces needs to be grinded smoother than average for compound gaskets, and the copper gasket needs regular heat treatment with a torch to soften, and still to positively ensure a gas tight seal, needs to be sprayed with a thin layer of copper or aluminium containing paint or grease. 

In my opinion, the basic fault with blown gaskets and those multiple retorque needed on our engines is the short, thick ungiving head bolts used. The only give there is, is in the thin material at the tip of the threads. It is a construction error or compromise done back in time when engines had to sustain service by anyone with a spanner. At the low combustion pressure there is in the flathead, the pressure of the bolts on the head gasket is many units (tons!) above what is needed to ensure a seal. With a bit of engineering, head bolts with a narrower waist, made of modern material torqued with a angle degree torque gauge to a calculated bolt stretch would solve the problem.