With the mitered corners, I would make a couple small fish plates and lay them inside the angle to reinforce the joint. Your gonna have alot of weight sitting up there and moving down the road is gonna make it even worse for wear.
Just my 2 cents.
I always go for the over build.
The mitered corners do have a nice look to them tho. I like it.
Go Big or Go Home.....Thats what I say
.One of these days I want to learn to weld real good
.........Sometimes I feel like a nut.