The 12 valve (1989 to 1997) found in Dodges... (or school buses until 2002) has either the VE Rotary injection pump (1989-1993ish) or the P-pump(1993+). Both of these pumps RELIABLY can make additional horsepower with just a flathead screwdriver. Add a governor spring into the VE pump, or a fuel plate into a P-pump and it can REALLY wake up. (500-600 ft lbs easy).
These pumps (12 valve motors)also require only power to a fuel solenoid and the starter. They are easy to work on, easy to wire, easy to maintain and are known for reliable fuel delivery.
The 24 Valve came about with the VP44 injection pump in 1998-2002. VP44 is known for higher failure rates, not easily modified, and fuel conditioning (filtering) is VERY important for this pump to remain operational. Wiring, throttle position sensing, and emissions start to play a role on these motors. These pumps (VP44) also have fuel delivery ceilings MUCH lower than that of its elder 1994-1997 P-pump brother. IE the P-pump can actually make MORE power than a VP44. (So its less reliable, harder to modify, harder to maintain, and actually makes less power when turned up). VP44 injection pumps in 24 valves are known to be thrown away and guys will actually swap a 12 valve P-pump injection pump onto a 24 valve motor.
After that its an 2003+ ISB motor which is DEFINITELY more wiring complex.
My 1993 Cummins 5.9 "6BT" with the Bosch VE is known more for MPG that its P-pump brother, but "stock form" is 160HP, 420 ft lbs of torque. I have turned up the fuel screw a few turns and an updated governor spring installed. I will be around 200 HP and 500 ft-lbs of torque. All while pushing the M715 around at probably 15-17 MPG. (which is good for our heavy lego-block trucks)
67' M715/5.9 Cummins/ZF5/NP205