Forget it appart from the mechanical and electrical problems you would have a car that was not type approved therefore you would be unable to tax and insure .

If you look on the internet you will a few examples of 405s which have had the 406 3.0 V6 transplanted into it. Companies who offer the service charge many times what the finished car is worth. Furthermore the 406 V6, whilst having a lovely engine, didnt handle nearly as well as the 405, so adding that heavy engine to the front of a 405 wont make it drive better, even if it goes fast in a straight line. And yes, having swapped the engine and gearbox, you would also need uprated cooling and brakes, and suspension.

If you want a faster 405, buy an MI16, they arent slow. If you want it to go faster still, the MI16 engine can be tuned a fair bit.