Of course it can handle more than the rating, because all engineers, you and me included, operate using a factor of safety. This is used to compinsate* for assumptions made and the fact that empirical formulas derived for the design are close approximations of actual forces incurred in the material.
You are suggesting that it is a good idea to reduce that factor of safety. This is not a great idea.
Now I do understand that assuming that the brakes and frame and all other aspects except the motor are the same than you should be able to stop and manauver the load safely.
Now, the 6,500# for the V6 is also based on having the stock weight distrubuting hitch reciever. Most do not install these on the 4 banger as they do not need them. Also required to tow the 6,500# is a brake controller also not commonly installed on the 4banger.
So, if you go ahead and get the weight dist. hitch and brake controller than your frame and brakes should be able to handle the load.
However, having towed some pretty heavy loads with my 4 banger, I would not want to even try to tow such a heavy load, and I would definatly not let the RPM's slip over the red line, and would not even consideer holding them close to the red line.
Just my 2 cents.