On the chemical forefront, I'd be attempting to break it with Aerokroil, but they do not give that stuff away (it's $26/can at my store). There's another product I carry called "Aero-Ayd", made by 1st Ayd, but I doubt that's available north of the border. It's not really a retail brand so much as a shop-supply company, but their stuff is so good I sell it over the counter.
Looking at things more homeopathically, I'd be sitting there with a large block of beeswax and mild heat... just enough to melt the wax and sweat it into the joint, none of this "glowing orange"nonsense. Propane would be more preferable to oxyacetylene in terms of temperature control. Done correctly (meaning time allowing) this is probably the most-effective way to split the two. On a joint that size, though, it'd take awhile to get it into the entire surface. You'll still need some real force to move that joint, though.
Another idea would be thermally shocking it. Heat the outer casting as evenly as possible to as high a temp as your comfort (and the casting) will withstand. Spray the inside of the upright with CO2, refrigerant
(I did not just say that), nitrous oxide, or some other highly-compressed gas that dispenses at extremely low temperature to shrink it quickly. Propane or oxygen would be a bit too dangerous for my tastes; N2O would probably be ideal because you'd at least find your total failure hysterically funny in the moment.
If you have a press into which it can be fit, I'd probably suggest that. Support the casting and put force on the upright. Don't go crazy, just put enough pressure on it to keep things tight. While you're doing other things, occasionally check on it and apply a bit more force. Optimally, you'd be doing this in conjunction with one of the previously-mentioned methods (I'm pro beeswax here) since there's about two square kilometers of surface area you need to overcome in that arrangement. I don't think simple chemical solution is going to be enough to do it without force applied. That being said, make sure the press is pushing perfectly in alignment with the centerline of the upright, which will require some creative supporting under the casting. Any off-axis force will serve only to potentially crack that casting.
Muriatic acid would absolutely get it apart, but would likely attack that casting like a fat kid on a Pop Tart. It would require your undivided attention and constant intervention--apply, wait a few minutes, flush, repeat. Not fun, not easy, not particularly safe for you, and potentially ruinous to the components. I got pistons out of two engines that way, but obviously the pistons were gar-
bosh and in one case, a couple of the cylinders will need sleeves.
