Alrighty then! Here's the diagrams in the book.
I was too lazy to scan it, but not too lazy to use the digicam
I was fairly certain before I looked, but the book confirmed what I thought. The doors were specifically built to incorporate the B-pillar, so they're not bolt on or weld on. You could in theory take a 240 coupe door and the vert door to someone and ask if they can combine the two. The only problem is that if you use a normal 240 door, you will loose about 4 or 5 pieces of bracing and reinforcement that Nissan put in the doors.
On the plus side, it would be a good time to consider a 4 pt harness if you decide to lose the B-pillar mounted 3 pt belt.