[mickmate]

Preheating is a good idea anyway, aluminium is such a good thermal conductor it sucks the heat away from the weld causing cracking at any major heat difference areas. If your welding a sheet to it, keep it thick enough to not burn it up. TIG is helpful with different thicknesses as you can direct the heat more where it's needed. Another important consideration is getting it clean, clean. With MIG it should be pretty much blasting its way through the aluminium oxide anyway if it's set for enough penetration. That should work if you can't bracket off the carb flange or similar.