Putting new cedar fence 4' high and 6" wide and 5/8" thick what type of fastener should we use I read somewhere that it should be either polymer coated or stainless steel or aluminum what is your preference among 3? also, I will use screws ...what size screws should we use?

Last year I built a large sliding gate to get a car through, something that's common in this area, although it seems nobody really knows how they should be built, they just copy existing gates. It slides across a header beam on a rail. Photos here: [url]https://www.icloud.com/sharedalbum/#B04GWZuqDGnHZvX;A77AFDA5-54D3-43D3-86ED-419DEE3CF777[/url] So anyways, I installed 2 2x8s as the header beam all the way across, which is the biggest size I've seen anybody use. I notched the 6x6 posts out by 3 inches at the top, and then used lag bolts to attach the header boards to the posts. The hardware guy said I could just use these thick 2 inch screws (or bolts, I forget what they're called, they're thick but pointy on one end) to attach the rail to the header. In retrospect this meant that only a tiny amount of each screw was actually going into the 2nd 2x8. This spring that came to my attention, as I noticed that the two header boards were not really joined properly, there was a gap between them in some spots, so it wasn't quite clear to me if the gate was mainly just hanging off the front board and not benefiting from the strength of the 2nd board. So I lagged them together using a couple of lag bolts in the middle of each span, top and bottom. Is that adequate? The spans are 11 ft each. More concerning, the corner of the gate had sagged downwards a bit. On closer inspection the header boards had cracked and had sagged forward a bit at the top. One potential issue is that the post had been a bit short on that side, so when I lagged the header to it, I had to do so in the middle of the header boards, rather than at the top. I don't know if that allowed the top of the board to pull forward and crack, or what. Another consideration was that when I notched out the post I hadn't done so perfectly. The cut sloped downward at the front, so that allowed the boards to hang forward a bit too. You can see in the photo I stuffed a piece of wood in there as a temporary / semi-permanent fix. Do you have any ideas for how to fix this problem, or how concerned I should be about those cracks in the header? The crack on the back runs the whole length of the header. The one on the front is covered by the rail but from what I can see I think it stops after a bit. I'm considering attaching an extra 2 inches of 6x6 post to the top of that post, to get the proper height (using mending plates to attach it to the post) and then running another lag bolt through that addition and the top of the header boards. Worth it? Thanks in advance for your help