Where you live, are you sure that you even need a heated manifold? Whatever you do, that piece is too rare to modify by welding. What about fabricating a tube the full length of manifold, water in/out at each end, form or shape it to closely conform to manifold, locate it under manifold towards engine side (somewhat hidden) and attach with lower carb studs that protrude all the way through attaching to tabs and held with nuts. I would think that possibly heat conduction/radiation will suffice. A drilled block of aluminum something akin to a six/eight carb log fuel manifold.
