If you are using straps, a shackle might be useful, but a chain is much better- it doesn't cut or wear on rough surfaces like a strap.
I would put on 3/8" chain hooks- that way you can use 3/8" chain OR 5/16" chain. Of course if you never plan to buy or use 3/8" chain, go with the smaller hook if you want.
I would place them at the center line of the hydraulic lift arms. THE best way is to just use a sling from each hook to lift your load but sometimes you have to do center pick. If using them to center point lift make a Y sling by attaching a chain to each hook, use on hook to attach back to the chain forming the Y, then attach the end of the chain to your load. DONT lift heavy load from one side of the FEL. IT is OK for really light loads to lift from one hook, but a heavy load might torque your loader arms and/or tip your tractor on that side so be careful.