First thing I'd try would be to give the crankcase at least a 48 hr. soak in the hot antifreeze, to (maybe) remove any corrosion. Liquid Wrench, etc., might also help.
I have made a simple expansion thingy to remove rear ball races before. I turned an aluminum plug to a good fit, drilled a hole down the center, split it with a hacksaw, and screwed a sheet metal screw into it to expand it. Then, tapped on the aluminum slug, with heat on the case, and out she came. Same approach might be useful, possibly with a fender pounder's slide hammer attached to the sheet metal screw.
I had an OS Max III .35 with a suction insert that was solidly stuck in the venturi. I screwed an appropriate sized tap into it, pulled with visegrips, and out it came. Of course, the insert was trashed. A replica insert could probably be reverse-engineered without a lot of trouble. It depends on how serious you want to get. Why not try an 11-4?
Steve