I also think that 38 m wingspan is unduly restrictive.
Consider the existing big narrowbodies:
Airbus 321: wingspan 34,1 m, area 123 sq. m, MTOW up to 93 t.
Boeing 737-900: wingspan 34,3 m, area 125 sq. m, MTOW up to 79 t.
Boeing 757-300: wingspan 38 m, area 185 sq. m, MTOW up to 122 t.
Tupolev Tu-154M: wingspan 37,6 m, area 202 sq. m, MTOW up to 100 t.
Tupolev Tu-204-220: wingspan 41,8 m, area 182 sq. m, MTOW up to 111 t.
Boeing 707-120B: wingspan 39,9 m, area 226 sq. m, MTOW up to 117 t.
Boeing 707-320B: wingspan 44,4 m, area 283 sq. m, MTOW up to 151 t.
Douglas DC-8-73: wingspan 45,2 m, area 272 sq. m, MTOW up to 162 t.
Vickers VC-10: wingspan 44,6 m, area 272 sq. m, MTOW up to 152 t.
Ilyushin Il-62MK: wingspan 43,2 m, wing area 280 sq. m, MTOW up to 167 t.
For comparison, some small widebodies:
Lockheed Tristar-1: wingspan 47,3 m, wing area 320 sq. m, MTOW up to 195 t.
Airbus A300B2-200: wingspan 44,8 m, wing area 260 sq. m, MTOW 142 t; A300B4-200 MTOW up to 165 t.
Airbus A310-200: wingspan 43,9 m, wing area 219 sq. m, MTOW 142 t; A310-300 MTOW up to 165 t.
Boeing 767-200: wingspan 47,6 m, wing area 283 sq. m, MTOW 136 to 143 t; 767-200ER MTOW up to 176 t.
Ilyushin Il-86: wingspan 48,1 m, wing area 320 sq. m, MTOW 208 t.
So, summing up: what do you think would be the most efficient way to design an airplane with 120...150 t MTOW and 200...250 seats? A short widebody, like A300/A310/B767 non-ER, or a long narrowbody like 757-300/DC-8-61/63?