Steve,

Thinking about your project a little more, I would choose to alter the
design rather than trying to add a stand-off. There are simpler and
stronger designs possible, such as this for example:

http://www.hi-techedu.com/products/hfc14.htm

And here are a couple of older cranes which might give you ideas:

http://www.tclayton.demon.co.uk/pics/canal/npb5.html
http://www.stroudwater.co.uk/cpsn/dudbridge%20crane.jpg

Note that both of these have a compression member which is at an angle
to the vertical. This is a nice, simple design. You just need to avoid
applying large moments to the beam as in your original proposal. In pure
compression 2" x 2" x 0.25" tubing should be fine.

The most critical part of the crane is likely to be the point at which
the top is attached to the wall. If you choose to use expanding bolts,
look up their load rating and use a good margin of safety (a factor of
10, say). Use the right size drill and make sure the wall isn't crumbly.
Should it fail, a joint like this will not fail in a nice way.

All the best,

Chris