Allen, I am certainly not an expert but having just gone through this last week let me relay what I learned. Vintage rules say each leg of the slings must be able to hold the entire boat. In other words if you have 4 legs on the sling and your boat is say 1650 lbs then the safe working load of the sling must be 4 X 1650 or 6600 lbs. Here is the catch, slings on a boat are not usually loaded on a straight pull basis, that is vertical to the axis of the sling. They are lifting on say a 15 or 20 degree angle perhaps more. The sling builder will derate the capacity depending on the angle of the "choke". Thus if you are at 1650 with 4 choked lifting points the slings will need to be even greater SWL.
The slings we had built had webbing of 1" wide on one set and 2" wide on the other. The 1" was rated at 1600 lbs and the 2" at 3200. That is SWL not breaking strength. To get the rating required for our boats the builder doubled the thickness giving each leg a 3200 or 6400 lb SWL. We had him use webbing in multiple thicknesses as an endless loop at the crane end of the sling so we weren't trying to set a 10 lb iron loop on the decks. He then attached a label that gives the load capacity for the sling in total and another for each leg. This was date coded.
As often as necessary they will test to 2X the SWL and measure elongation. The testing methods are set by the American Society of Testing Materials or ASTM. As our slings get literally zero work and aren't subjected to chemicals, sunlight or abrasion they should last forever.
To find the boat weight just about any large gravel operation has scales that are surprisingly accurate. We already knew within 1% our trailer weight as here in NY we have to have them weighed to get a license. Then back to the pit with the boat on the trailer, some subtraction and you have the answer. The pits have a licensed "Weigh Master".
To find a place that does the slings we looked in the Yellow Pages under Material Handling. Voila.