Be VERY careful. That nice new radiator is put together with soft solder. It melts WAY below brazing temp. Make sure that you have something between the core and tanks and where you are brazing to absorb the heat. Lots of wet towels maybe. Watched a fellow solder up a small hole in the tank on a top tank radiator one time with a propane torch. Got the hole soldered up fine. But there was nice shiny solder dripping out from under the edge of the tank when he got through. Leaked all along the seam when he refilled it.
Would have been ok, if he had taken it to a radiator shop then, but he tried to resolder the tank......
Radiator shop could not even put all the pieces back to together.
If it is long enough, sand the inside and out side of the end. Carefully crimp it almost shut. Swipe a thin coating of flux inside and finish crimping. Put lots of wet towels around the tank and between the tube end and the tank. Heat about 1/8 inch back from the end with a propane torch, and then swipe the edge one time with a thin stick of 50/50 solder. If you have it hot enough, it will melt and suck right in. Wait about 5 seconds, and then pour water on the seam.
J.