Dir-T-Birds, here are the numbers I came up with based on your dimensions and if I am correct in that your using pipe for your firebox.
A 4" ID stack of 43.20 in.
You will need 33 Sq.in of intake opening.
Your firebox to cooking chamber opening will need to be 87 Sq.in. A half moon cut 15" x 7.5" will give you approximately 88 Sq. in.
You want the R/F plate opening opposite the firebox to at least equal your firebox to cooking chamber opening.
The clearance between the R/F plate and lower grate can be as close as 1" or as far as 5". A Lang starts out at 1" on the firebox side and falls approximately 3'' away from the firebox. I built mine with a 4" clearance and a 1" fall making the far end right at 5". I hope this helps.