Getting silver solder to stick to stainless steel and cast iron at the same time is difficult, if not impossible. Brass would be much simpler to get a water-tight bond. Of course, green LocTite "stud and bearing mount" along with a snug press fit would probably work as well as silver solder, and be easier to apply. It's stable up to 500+ degrees F.- - -far hotter than a sleeve exposed to coolant would ever get.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!