is one brand, but most local welding shops will carry some version of the stuff. I used it to braze my intake tubes to 1/4" plate, but I have no idea whether it will stand up to the intake pulses & vibration with the engine running. I can tell you that the stuff isn't the magically easy to use stickum that it seems to be, when you're watching the carnival barker at SNF. The stuff I used was marginal on thick stuff with a propane torch (might not be a factor with the heat exchanger). You'll almost certainly need a wood handled stainless steel brush to free the corrosion on the base metal while the torch is applied to the molten brazing material.
Have you thought about hitting Barry up to take a look at it? ;-)
Charlie