Your getting close to 4 meters with a 500Kg load. You need to hire a license engineer. It will probably cost you 4-8 days of your salary. Even if the masonry column is .5m sq. I don't see a safe way to transfer the forces without having extreme detail about the column, and what it is holding up.
I tried to push you in a safe direction.
Many years ago I went down this same path of a jib crane, and adding it to a building after the fact, is just too much effort to properly support it. The other (safe) alternative is a bridge crane that is fixed and uses added steel support columns. That is what I ended up building. I used very light steel wide flange beams that were 6" tall, by 6.8lbs/foot (may be 7.2lbs/ft). This made 8 foot sections liftable by ME on a ladder. I used 2x4 steel tubing 1/4" wall thickness for the support columns. The columns have a welded plate on the each end, one to bolt to the floor, the other to bolt to the bridge sections. I have a 9/16" piece of round stock welded to the top of the bridge for a rail guide.
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