My idea of "street cars" with big block involves a boatload of nitrous and single digit time slips.....
I also like low sitting cars trucks. That being said id trim the top of that bracket until the box tube was equal with the top (if that makes any sense)
Also, pay attention to the ride height of the shock when setting this up. Make sure the brackets on the rear housing give you a lot of adjustment without lowering or raising the truck. Down the road if you get serious you will want a 15" ride height shock in there.... Sounds like a cool project