This is going to be hard to explain, so please bear with me. I am shrinking down an old 8' sliding glass door Rough opening from what appears to have been an old garage door originally. The header seems sound, so I plan on framing in under it, about 24"x 80" between the foundation edge which was also the old rough opening edge, and the edge of a the rough opening I want. As this is for an exterior door I need a minimum clearance of 6" vertical before the bottom of new siding, to prevent water and other issues. I plan on building a mold to pre-cast a piece of concrete I have designed that will have embedded threads for the sill plate and holes for rebar to be epoxy fixed once in place and connected to existing structures.
My question is this, should I design this to sit on existing slab and set in place with hydraulic cement (my current plan) or should I cut out a chunk of slab and design this to fit down next to the slab?
Remember, this will not be load bearing as it is under the header for the old door. As such do I need to worry about 'punch through' with the new block as it will not be over a footer as far as I know.
Thanks for reading and thinking about this.