1 3/4" Headers Verses 1 7/8" on a Healthy 408....Is There a Big Difference??

I'll just tell you this. When running examples through pipemax, the trend is that the more efficient the engine (VE) the smaller the pipes needed, all other things being held the same.
The other thing to look at is where the power band is. The higher in the rpm range that the engine runs, the larger pipes it will prefer.