I'm sorry, I was confused because I'm on dial up internet and the link didn't completely load. All I was getting this morning was the top picture and I couldn't figure out what the 75" mattress was about. The twin bed mattress is 39"x75". If the width would allow I would make the space between the side rails 40" and the length between the head and footboard 76" allowing an inch for bedding. Rather than mortising the slats into the side rails like shown I would just put an strip of oak 3/4"x 1 1/2" at the bottom of the rail, the full length of the rail. Since there is no box springs I think I would make the slats out of hardwood as well and screw them to the strip of oak on the side rail. That way if the children start jumping on the bed the side rails don't defect outward and let the slats fall. Depending on how ridgid the mattress is you might need to put a piece of 1/2" plywood over the slats for more support. I think you are right about the 3 1/2" side rails. I think they need to be 5" to 7" wide even using oak.