modified firing order big block

There is some merit to the swap - provided you use it in the rpm ranges where it might help. My understanding is not complete on it - but it seems like it reduces the harmonics and pumping losses from the opposite cylinders firing. In doing so it frees up a little power. I know the FAST guys use the swapped order. I can't say as it's worth a ton of power - but because it's "free", meaning you're simply reducing the parasitic loss, you don't have to make more power to get the bonus of having it at the tires. I'd suggest you post over at Speedtalk.com - there are pros there that make use of that swap.