There is nothing magic about the sheetmetal in your old furnace. If it was manufactured before, it can be welded or brazed. the problem is not in the sealing of metal to metal. The problem is who is going to do the job and who is going to take the liability. If you do it yourself, you take the blame for cracking during the heating season.
In our litigous society, a heating contractor takes the risk for old equipment. In this case, historic antique equipment. If you can remove the heat exchanger and have a welder do it outside the building, then reinstall it, there might be less objection.