Really, the OM engines just do not have the grunt to get a vehicle this size up to a useful speed. IF you installed one, you'd need like a 4.56 diff gear, and the engine would never reach 50mph...
keep in mind the tire size difference on the truck vs the car the OM came in... factor in the gear ratio, and imagine what you'd need to drive the truck...
603 engines came stock with 2.65 ratio gearing, the sub likely came with 3.42 or 3.73, and around 300 hp... 200 or so lbs torque... the ONLY engine I'd consider for this truck would be a TUNED 603 with a 5K reworked IP, and a monster turbo, or a 98plus 606 with a reworked IP from a 603 to get out of the electronic stuff...
then imagine all the stuff you'd need to do to get that to fit...
then think on the simplicity of just pulling the anchor from your current truck, and simply bolting in a 6.2 or 6.5
you do the math.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!