QV- Thanks for including some shots.

A few comments- can you see the SS/ST crack inside the ST? Reason I ask is that the ST looks to be very thin at the top. It's a known concern to weld stays to a thin ST, this is why so many ATBs have reinforcing sleeves or butted ST top sections. It's quite possible that the weld it's self didn't fail but the tube cracked right at the weld edge, especially if the ST had been reamed after the welding and further thinned right at the weld's inside the ST spot.

It looks like the ST/post clamp section got pried open a bit. Are the binder ears still well joined to the ST? If so then careful reshaping the clamp in general would be the first attempt to restore this aspect. This reshaping will need to be done if a collar is added anyways. But until I try the existing binder I'd not assume the need for an additional collar.

Silver 56% is not meant to be used in a fillet situation. It looses strength as it's thickness grows. It wants about a 0.005" gap max for full strength. When filleted 56% will cool first on the outside of the fillet, then as the inside cools it contracts a bit. This induces stress and a possible interior crack to the fillet. Additionally in general any silver filler wants a very clean surface to be added onto. Bronze if far stronger with gaps and globs (which is what a fillet really is, a nicely shaped glob) and is far more tolerant of less clean surfaces. Unless you have a sand blaster getting a weld to be fully clean for silver takes a lot of work and often some weld bead removal to get the low points and tiny pocks clean. Silver can be used with far more success as a file scratch filler on the surface of a lug or tube. There's no real stress for any preexisting stress risers to grow from.

I'm no welder but the welds on this frame look pretty lumpy and ragged to my eyes. I know that's no true indication of how well the weld fused the two tubes (can't tell a book by it's cover). But taken with other aspects it does lead me to look at this frame with a questioning eye. Ugly and uneven welds, very thin ST top section, chrome that has easily flaked off (and chroming any frame needs care to prevent additional internal corrosion concerns) (was the chrome even applied over a copper then nickel base?) all are trending in the same side of the quality fence.

It is good that you are taking the time to do your homework before just diving in and doing stuff that isn't likely to work well and might make the frame's integrity worse. Andy.