Did he ever run the 383 with open exhaust? Most likely the 383 would not pick up as much. Some of these guys that think a 383 is a better motor than a 440 are off base. Your buddy made the right move. Dollar for dollar the 440 will out run the 383.
The exhaust is what is restricting your 440. The 440 is exhaling more than the 383.
I have full 3 " exhaust on my 850 horse Cuda and uncapped I only pick up less than a tenth.
If he wants to run through the exhaust he should start by upgrading the exhaust.
By the way I am running Magnaflow mufflers. They are straight through.