400 series is 2 1/2 T. 402-27 149" WB. 270 Motor. Big MoFo. 18,000 GVW, 30,000 GCW. 15,000 Rear axle, 4500 front axle.
Looks like the motor number was used instead of the Serial as many MVD's did. Was not a good idea because motors got switched out. Your motor is probably original. Allegedly the block casting number can identify the era, the head casting no. can identify the year. Could be date codes also. Not sure about GMC.
