I went 3" with a mild 413 in my challenger, simply because it didn't cost that much more over 2.5 and would be one less thing to change when I go more motor.

Far as price, hard to beat the systems jegs/summit sells, which are for headers, not sure which you'd need.

I have aluminumized on my cummins, 2 winters with lots of salt, no rust even showing on it yet.