On the underside of the benchtop, the top of the sliding deadman will slot into a groove. I marked out the groove to be 1″ thick based on the size of my chisel, and bored out most of the waste with a brace and bit.
On what will be the right hand side of the bench, I bored out a deeper section where the deadman can be inserted to clear the lower rail upon which it will slide. This way the top of the deadman will be flush with the benchtop, as opposed to having a clearance gap. Maybe I over thought this process and the entry mortise was more effort than it was worth. Time will tell.