JB weld is just epoxy resin, with aluminum powder mixed into it to make it look like aluminum when it cures.....
JB Weld does not really work well to fix things like this. It works best as a material to plug holes on "non- stressed" aluminum items (like motorcycle engine aluminum side covers or oil pans). Holes that do not involved the high tensile stresses as would be present in a crack like the one on your Guerciotti's head lug. Only way to fix it would be either to replace the lug with a new one or weld the crack with a full penetration aluminum weld, both methods that require the services of one with the experience and expertise to do so......certainly not a DIY project for most of us.
Alan used to provide frame repair services that includes replacement of cracked lugs, but I heard that they stopped doing so some years ago (I'm not even sure that Alan still exists as the same company out there).
Last edited by Chombi; 03-22-14 at 02:09 PM.