I have dimensions for the full size Cassutt Special I and smaller Cassutt Special II, but not for the IIIM. However, I have 20 photographs here of Cassutt IIIMs, with some detail shots showing tail and wing, etc. Perhaps they might be somewhat helpful. Airmark Ltd. in the U.K. built some production Airmark/Cassutt IIIMs, these were slightly modified from the Cassutt Special Is.
The Cassutt Special I used a Cassutt 1107 wing airfoil, no incidence or dihedral. The Cassutt Special II used a Cassutt 13106 airfoil, no incidence or dihedral. The Cassutt Special IIIM airfoil is unknown to me. Various wingspans were 13 '8', 14' 4", 14' 6", 14' 11" and 15' 0". Overall fuselage lengths varied at least as much as a 1' 3" difference. Height overall also varied from at least 4' 0" to 4' 11". Many builder-modifications exist. If you could find the IIIM dimensions you could then scale your model.