I visited a local engine rebuild shop this past August (bought a crankshaft from them). While there I talked to the shop manager about rebuilding 6.5L cylinder heads. One of the items we talked about was valve guide liners. This is the same shop that rebuilt our 6.5L TD 18:1 CR project engine.

I mentioned to the manager that cast-iron valve guide liners were being recommended for diesel engines because of the abrasive nature of soot. He quickly told me that they use bronze (probably the phosphor bronze variety), and that there hasn't been a problem with accelerated wear in these light-duty diesels. After considering that our 6.5L project ran to 300K+ miles, their advice is likely pretty good. However, he did say that they could do cast-iron valve guide liners - they're just quite a bit more expensive.