I'm not sure how the topslide fixes to the cross slide on your machine but if there are some holes in the cross slide you can use these to mount the casting with suitable packing between to bring it upto center height. The hole can then be bored with a simple home made between centers boring bar and a flycutter used to face the cylinder mounting flange at the same setting to keep things true and square.
Another alternative would be to rotate the vertical slide through 90degrees if you can and then hold the base of the casting against the slide. Machine as above.
How much room is there between mill taple and the spindle nose, another option would be to mount casting with flange facing upwards on the mill with an angle plate and use a boring head to do the hole and flycut the flange.
J