Exhaust: MPG vs. Torque

Guys,

As always, this board is incredibly helpful. I did a search on exhaust systems, exhausts, etc. and got many of my questions answered. But a few have been left open for consideration. So, I’d like to see what opinions people have on the following.

I’m getting ready to (re) build the exhaust system for my ECJ5. I have a Chevy 350 and will be running block hugger headers with pipes/mufflers inside the frame, out to the back. I learned from this board that turning the exhaust tips out at 45 on the rear corners and down 30 degrees will limit exhaust fumes from entering the cab at speed. (In case you’re curious, I had fender well headers mated to turbo mufflers and exhaust tips turned out in front of the rear wheels. Too load and smelly for me now.)

Objectives in rebuild (in priority order):

1. Improve MPG
2. lower noise (tread lightly – also means tread softly in my book). I know choosing headers isn’t exactly the step in the right direction for noise control – but I had to use these for clearance issues – stock manifolds wouldn’t work in my set up.
3. retain torque

I’ve read on this board that bigger pipes mean better fuel economy. And smaller pipes mean more torque. If I had to give up some torque for better MPG, no problem in my book. But I’m wondering if you all have any opinions on my build – So…

Should I go with a 2” OD pipe from Headers back or a 2.5” OD?
Is there a measurable difference?
Which would do better at reaching my objectives?
Also, if any of you have muffler comments/recommendations, please provide.

Thanks,