A lot of people have expressed concern about the possibility of tipping. It's a legitimate concern and anyone who doesn't understand the reason should not attempt something like this. I thought I indicated an adequate awareness in the initial post.
In using this set-up, if I needed to move the chuck to the other side of the shop, I would lower it within 6" of the ground. If the crane tipped, it wouldn't be a big deal. If it felt unstable, I would add counterweights.
Nearly every day, I use tools to do things different from the intended purposes of the tools. Sometimes it is because I need to accomplish a task now, and I don't want to wait a week for a new tool to arrive. Sometimes it is a one-time task that doesn't justify a new tool. Sometimes there just isn't a tool to buy for the particular job. I think this is true for most people here. This crane adaptation was made from scraps and took less than 2 hours of my time. I think the cost/benefit ratio is pretty good.
Everything we do involves some sort of calculated risk, even if we are not conscious of it. If your use a pencil in an unsafe way, you can blind yourself.
When I cautiously lifted my largest chuck (170#) for the first time, I tried lifting up on the back end of the crane to see if it would tip easily. With a lot of force from my legs, I could barely get the wheels off the ground. That seemed fine to me. I could easily add counterbalance, but that was my heaviest chuck, so it seems like overkill.
Out of curiosity, I did a load test. The rear wheels of the crane begin to lift with a 450# load.