Yeah... I've done two of these mods and used silver solder...
just because that's what I had and used on these sorts of pipes. I also used MAFF gas and not Propane because it allows you to heat it up faster and with less contamination. We also made up the fittings for the front end OFF the car measuring and aligning them to the cut off sections. Then slide them in, and soldered the rear end sections ON the car. Made it much easier.
(Still easier are two straight pipes and one or two long sections of flex hose... Its been done successfully!)
Also... I just bought the copper and fittings from Lowes and did not notice a CHOICE in types.
As for vibration and stuff... Has proven NOT to be a problem in the last 10 years or so.
Photo of my "boy" Tommy at the beginning of the surgery...
Photo of our end result! HA!
When we pressure tested the system we found a weak spot on the radiator outlet nub... and we were too whooped to drop it and fix it.
Called for AAA and had Mike and Big Bird hauled home some 60 miles and he took it to a radiator shop the next day.
But since then there were no problems and the cooling system has worked flawlessly.
HTH...