Sherlock - yes, solder would provide more than enough strength for a top on a block if the surfaces were properly prepared, the strength issue was mentioned regarding Jessm's radiator fins
And yes, a much higher temp is required, but that is a result of the metals in the compounds and their melting points.
Jim - brazing copper is
very common is quality heat exchangers for the attachment of the fins to the tubes and is used for the 2 reasons I stated (much better thermal conductivity and extra strength, tho mostly for the first reason). You can get silver brazing compounds in paste, bar, and wire forms. There are different compounds that have differing heat ranges requirements as well. Just be sure to
never get the older types that have cadmium in them (extreme health hazard).