I got to work on it for a few hours tonight and made some good progress.

I got the fenders and doors removed, along with the steering column and everything removable in the dash.
Tomorrow I'll start bracing it up. I'm thinking like this:
Initial cutting should be started Saturday. I'm thinking like this:
and
At the moment I'm planning on leaving a good amount of the floor in the back with the body. The opel is about 12" longer behind the rear wheels than the pathfinder, so I want to leave some extra floor until I decide where I want to have them mate up.
I'm also going to try to keep some of the structure around the engine bay so I can maintain the grill area for now. I fully expect to have to cut it off once I try to mate them together, but I want to see where things land before I decide where the cuts need to be.
That reminds me, I'll need to add a brace along the bottom of the front wheel wells in order for that to work.

Meh, if it doesn't work out, I'll just cut the grill support area out and add it back to the pathfinder as needed.