I have found the drawing of the Muncaster engine edited by ET Westbury, a great piece of machinery and a impressive model, but dimensions are scarce !
It is very tricky to optimize lever length to minimize curvature of the path of the crosshead between 2 antagonist arcs, too many variables for my old brain, and I don't have access to artificial intelligence...
PS : a slot in a linkage releases a constrain and would permit a linear movement of the crosshead to occur, (if this part is already linearly guided), but would'not in its own guarantees a rectilinear movement, imho.