In a dual exhaust system a 3" exhaust is necessary when you start pushing 400 HP.
squid..your thinking is right on the money. I have 1-3/4" primaries on my 5.0, but I'm pushing 500 HP and alot more torque. The JBA 1-1/2's are a good header. I've seen alot of people have there torque killed by using a 1-5/8" header on an E7TE head. Well...good example...the car in my sig is running E7TE heads unported with the old 1.78/1.45 valves and has a 1-1/2, 1-3/4, 2 to 3" step header....I just won't tell you the length
Murdock