Ford silver soldered the elbow to the thermostat flange originally, but the elbow to tank joint was soft soldered. If the joint at the tank to elbow isn't made properly, it will never stop cracking and leaking. It needs to have a flange, not just a hole. The elbow needs to fit snugly into the flanged hole. If the joint design isn't right, you can have a ring put around the tube and this will reinforce the joint. This is all pretty basic stuff that all radiator shops know. If a radiator shop is telling you that it is difficult to clean it up after it leaks, find another shop. I worked at a radiator shop for a few years after I got out of high school, and the first things they had me work on were surge tanks because they are so easy to repair and really hard to screw up.
I use brand new copper/brass surge tanks on everything I build, then modify them to work in the Cobra application. I have never had a tank leak. It's a non-issue for me.
Antifreeze gets into the metal, eh?
