I agree with Gordon on having an ECM, look at Keith's (rellick) Cummins/Allison combination. With a simple reprogramming of the ECM he went to maximum horse power and from a 5spd to 6spd transmission
. Detroit Diesel went to the 'silver' model in 1981 and DDEC 1 was introduced in 1985. My 1986 does not have DDEC, if it did it probably wouldn't be sitting in my driveway with a seized motor.