That's an important point! Even when using a single sheave block, the block will be carrying double the stress that the rope does. If you use a two sheave block then the load on the block is even higher. The angle between the incoming and outgoing line also affects the load on the block. The "Rigging" TM covers all of this. Unless you're well familiar with towing and rigging I strongly suggest that anyone contemplating any towing operation read and understand it first. FWIW, for my 6,000# M-37 with a 7,500# Working Load winch I bought a block rated for 24,000# load.At a minimum I would want to see the block rated for double the winch pulling capacity.
Also the diameter of the sheave in the block in important. A too small diameter sheave will damage the cable by bending it too sharply. The block that I bought has an 8" diameter sheave and I'm using it with 7/16" cable. Again the Rigging TM covers this in detail.
FWIW I haven't had any luck with AW Direct. Their cables are too short, too small AND too weak; even for my M-37. The stock winch cable on the M-37 is 7/16" diameter x 175' long with a working load of 7,500 pounds and AW's cables doen't come close to that. And I'd LIKE to have a 200' x 1/2" cable!