I'm getting ready to replace a leaking thermostat housing on a N18 engine with 155,000 miles. My question to those of you who are more knowledgeable, should I go ahead and also replace the coolant crossover pipe while in there? I see post that state the plastic pipe becomes very brittle after so many miles.

Honestly I'm surprised you haven't had to replace the thermostat sooner. But yes you're likely to crack the pipe while fiddling with the thermostat housing. It's not that expensive and beats doing the job twice.

I'm doing mine soon -- parts are on order -- and decided to do the water pipe. Considered doing the water pump too but decided to leave well enough alone.

Are you going to remove your intake manifold for the crossover pipe? I've seen some videos where they just fished it through with the help of a borescope.

I wasn't planning to remove the intake manifold. I don't think I've seen a video that shows removing the manifold. Do you have a link for a video that shows removing the manifold? I'd like to see what's involved just for another perspective.