The 400/500e's have a different firewall to give more space for a v8 engine. They also have bigger spindles, different sway bars,, hub bearings, a stronger rear subframe, and the bigger 1.3L differential. They also come with a rear mounted battery already.

The only reason I would consider dealing with a 6cyl car is because that's all the coupes came in.

But if 4 doors don't bother you, find a beater 400e.