Assuming your cylinder can be disassembled, the big reason not to do as you're thinking is because it's SO much easier to do internally with a simple spacer. It's considered a permanent solution in oem cylinder applications while external spacers are not. The answer you're probably looking for though is parallelism. Any misalignment or junk between the contacting surfaces will side load the rod, wearing the guides and eventually the seals. Eventually that junk and misalignment will wear and deform the contacting surfaces, reducing the surface area and it just snowballs into a failed cylinder. Any trapped fluid will be pressed into the seal. Depending on the style of cylinder you have, the end may not be perpendicular to the stroke and once you "run the numbers" you may not have enough surface area. Make sure you add in the force of the steering box if you're running one and the load coming from the tires when you hit something.
Granted it's only at full lock, so your idea may last an acceptable amount of time depending on your expected usage. You should be using jam nuts, so that's even easier than modifying the clevis if you decide to try it... Maximize the surface area, make it as parallel as possible and chamfer to an undercut where it's not contacting. May be worth looking at making it of a slightly softer material than the cylinder so that it's the sacrificial part.