You're right that Dell laptops are relatively easy to modify and upgrade - for a laptop. But still you can't expect to transplant a motherboard into any but the most closely related model. I upgraded my old M90 to an M6300 by replacing the motherboard, CPU and memory. For the M6400, I believe that the motherboard and case from the M6500 should be compatible (provided you change the CPU and CPU heatsink) but I cannot be entirely sure.
The newer 17 inch Dell models have 1920x1080 screens instead of 1920x1200. You couldn't jam the older screen into them because it is physically a different size, even if the connector turns out to be the same.