[MrE]

I had no problem fitting a Cummins ISM in my 1986.

Detroit Diesel 8V92T
approx.:
12.1 liter, 736 CID
length: 1219 mm, 48 in
width: 991 mm, 39 in
height: 1321 mm, 52 in
weight: 1086 kg, 2389 lb

Detroit Diesel Series 60
approx:
12.7 liter, 775 CID
length: 56.2 in, 1426 mm
width: 37.5 in, 953 mm
height: 49.6 in, 1259 mm
weight: 1270 kg, 2794 lb


Cummins ISM
approx:
10.8 liter, 661 CID
length: 50.8 in, 1290 mm
width: 38.3 in, 973 mm
height: 45.0 in, 1142 mm
weight: 940kg, 2072 lb

The ISM can produce 500 hp and 1550 lb torque at 1300 rpm. ISM data sheet.