How far are you planning on hauling those logs?
I see allot of steps here in getting them to the barn. You will load them with the grapple on your tractor. Just putting logs on the trailer will beat up a trailer over time. Then driving the logs to the barn will beat up a trailer. And getting the logs off the trailer might cause the worse damage of all.
I have a grapple on my loader and when I want a log near my shop, I just pick it up and drive it to the shop. One step and it's done. I only have 68 acres, so I'm not going that far, but I've piled up about 100 cedars this way, plus quite a few oaks, not to mention hundreds of trees that I've burned.
The trailer sounds like allot of work to build that wont be of much use.
Wait until you finish your grapple and see how that works for you. If you still need the trailer after you have the grapple, then go for it, but until then, I wouldn't build it.
Eddie