I have a 95 chevy P30 step van. I want to swap in a 98 12 valve cummins what wiring do I need for the swap?

 

You'd need the engine wiring harness from the 98.

If the 98 is from a different application, you'll likely want a wiring harness from a 98 in your application for ease of installation.

In the pickup trucks, 96 was an update year, and not everything is the exact same.

I would at least get the matching harness for your new engine, if not the matching harness for your engine and application.

 

Having torn apart a couple of 2ndgen 12v trucks... and seen the mess of harness they have... I'd wire that engine from scratch. All you need is +12v to a few places, and a voltage reg for the alternator.