Would it be hard?
 

I'm wondering if it would be hard/costly/time consuming to wire a 12 valve cummins?

Pre ecm would be pretty easy.

Wiring Harness

Nope.

1) Ignition hot to the fuel solenoid.

2) power to the KSB (cold start advance) through the temp switch (or a toggle switch).

3) power to the grid heater - if you care, or if you experience very cold climates.

:c:

Don't Forget the Starter
 

And it's even pretty simple with a 91.5-93 "PCM'd" manual truck. Auto is just another slightly more complicated thing needing addressed.