870 isx replacing a 871isx.

[Nostalgic]

I don't know a lot about how the 870's relay data to your CECU and gauges. I'm guessing, but I don't know how well that could be ironed out without a mess in your cab - I don't think the cabs had CECU's before '07 or the engine data didn't go through them? Maybe someone else can chime in here.

To use your 871 ECM, it need the exhaust pressure sensor (on thermostat housing - from exhaust manifold). To use the exhaust manifold, there's end caps without the EGR cooler bellow on ebay, but I think the 870 has a different turbo flange anyway, you'll need to use the center and rear section off your 871. As for the intake manifold, the 871 needs IMAP sensor - the 870 used separate temp and pressure sensors that screwed in, so you'll need the 871 intake manifold too...

Once you deviate from what left the factory, there's always catch 22's but anything can be done with enough time and money. I know that's probably not what you were hoping, that swapping in the running 870 would save you some of both, but I'm afraid that's usually not the case.