I've never disassembled a 700 car, but on the 240 models the wiring harness can be separated into a front section (everything in front of the front seats) and much smaller rear section. The emissions/fuel/ignition system and computers on the V-6 and 4-cylinder cars is different enough that you may need to do the entire front section - e.g., essentially swap dash panels as well as engines.

This is how I've seen it done when making the early, 240-based Bertone v-6 cars into four bangers. Good luck.