Have you considered welding a nut or a rod to the pin????
No guarantee but I would do some easy filling on the casting where the pin is located....just enough to see the outline/crack of the pin from the cast body of the transmission.....find myself a bunch of 5/16 nuts and weld down the middle...onto the pin......just like a broken stud........ a few will stick and a few nuts will fall off....and heat will build up in the tranny casting....... when you have a nut that seems to be solid....use penetrating oil or beeswax on the parts.... tap\ a few times on the nut......try moving the nut with a wrench....slowly back and forth..... it may break loose...start again...eventually is should release......
....and according to the drawing in your first posting the big pin is accessible from both ends....... can you tap lightly from each end of the big pin alternating every 3 or 4 taps..... it might loosen up the pin......also drilling the pin with a 1/8 bit then trying to screw in a metal screw to see if the pin might turn or even pull out....... patience is a virtue........ Good Luck
