Spreader beam

Here are some suggestions:

1. Borrow or rent a spreader beam from local crane company
2. Buy an off-the-shelf beam that has been engineered and tested such as:

http://www.buylifters.com/PDF/STRONGBAC/SpreaderBeams.pdf

The reference given by Guest is good. If you want to design it yourself, you will easily be able to see that the beam is primarily a compression strut and carries no bending.

Another option is a beam such as the one shown here. This beam is a double cantilever and carries bending moment but no axial force. The choice is yours.

But, if you have to design yourself ...........

The best resource is:

The AISC Engineering Journal, Fourth Quarter 1991 - 10 page article by David Ricker titled "Design and Construction of Lifting Beams"

Available for $10 (US) at:

http://www.aisc.org/Template.cfm?Section=Technical_Answers&Template=/Ecommerce/ProductDisplay.cfm&ProductID=861

Once designed, it needs to be built, witness-tested and certified by the surveyor-inspector of the organisation's lifting equipment insurance company at prescribed intervals, and maintained on the local lifting equipment register. Only then will it be covered by the insurance comapny's policies in the event of an incident or accident.

As far as lifting equipment goes, insurance is everything.