Hi WideMike:
I've done lots of them (not in USA) and must say I am surprised by the lack of codes governing the design of such equipment. So I think you're pretty much on your own with this one.
I'll assume you are lifting hardware and not human beings.
I use the code for the design of steel buildings. It is quite relevant and applicable.
Add at least 33% to your expected load to account for possible impact loading. Generally I'll add 50% or even 100% depending on how expensive the lift is. After that, design is straightforward using classic beam theory. Don't forget about possible axial forces, or else specify the slings to be nearly vertical.
Good luck and Merry Christmas.