Let it marinate a bit, but after that, drain it and break out the heat and a hammer.
Since the tube bundle is soldered together, you have a somewhat fine line to toe; you want to heat the cast iron as hot as you can without softening the solder in the HE bundle so when you start hammering you don't mess up the bundle. In my experience, the hottest flame you have access to is the best; the idea is to heat the cast iron evenly first, and then get the bronze bundle moving out before it gets as hot as cast iron housing. Ox-acetylene is best (but keep it moving and remember that the heat flows through the material; after you take the heat away, areas that are further inside will continue to heat until an equilibrium is reached), followed by mapp gas or a propane
torch (which may heat too slowly to be any use). Once the temperatures of the two become the same, you have to let everything cool down and start over.
Equally important are the tools you use to drive the bundle out. At first, you can just use a board that covers the entire end of the bundle to drive against, but if the bundle still has to be driven after it enters the heat exchanger
, you need to have what is essentially a huge dowel the same diameter as the bundle to spread the force of the impacts equally over the relatively soft end face of the bundle.
Also important is to have the housing solidly held, clamped or wedged so that it cannot move. This ensures that when you start with the hammer all the force is applied to moving the bundle out of the housing and not to bouncing the assembly around or distorting the bundle. Short, sharp blows are best here; we're not busting concrete.
It may be advantageous to try it from both ends, it is possible that it will come our from one side or the other more easily...