How is the powerband and mileage with those 500's?

who cares about mpg? It's a 4 4 o! and if you go 2x4's do it right and get a set of holleys.
No idea but if I had to bet 2x500 eddys vs a 6-pack, I'll bet on the 6-pack. Now a set of DP's and a good 2x4 intake I'll take the twin holleys