overriding the pin change interrupts for Arduino

If I want to write my own ISR for the INT0_vect or INT1_vect interrupts in Arduino code while eliminating the overhead of the generic handlers (used by attachInterrupt) in WInterrupt.c, how should I go about it?

Is it sufficient to declare the ISR in my code, assuming it will override the generic definition? Or do I have to copy WInterrupt.c into my sketch folder, eliminate the generic code and then compile? Or something else?

I’d appreciate any pointers on how to go about this.

I understand that once this is done, I will not be able to use attachInterrupt and so forth. I also realize that this need not be done for most applications, but for a program I’m testing where I want very accurate timing on the interrupts, the overhead of the test and function call seems unnecessary.