I found some info on proof testing modified lifting equipment.

http://www.hq.nasa.gov/office/codeq/...led/1740_9.pdf
Proof Load Test. Before first use, all new, extensively repaired,
extensively
modified, or altered cranes and derricks shall undergo a proof load test. A
proof load test also should be performed when there is a question in design
or
previous testing. Mobile cranes and derricks shall be tested at the minimum
working radius (and maximum working radius for new cranes and derricks
only) with a load as close as possible to, but not exceeding 1.10 times the
rated load at the given radius. The load shall be lifted slowly and in an
area
where minimal damage will occur if the crane/derrick fails. The minimum
radius/maximum load capacity of the crane/derrick shall be clearly marked to
be legible from the operator's or user's position and shall not be more than
the proof test weight divided by 1.10. For cranes/derricks with separate
lifting systems of different ratings, the markings will indicate the lifting
capabilities of each system (e.g., main hook, whip hook, and auxiliary
hook).
Proof load tests conducted by the manufacturer prior to delivery are
acceptable if the necessary test certification papers are provided to verify
the
extent and thoroughness of the test on that specific item.

My cherry picker is rated at 4000 lbs at the shortest boom length. I don't
plan to ever lift more than 1000 lbs if I modify the lift to a crane.
According to the above, for a 1000 lb rating I should test at 1100 lbs. If
I needed to lift heavier I would test to 110% of whatever weight I wanted to
lift. I also found some info stating that the minimum design factor should
be 3.0, doesn't sound like a bad place to start.

RogerN