Need to upgrade exhaust for street/strip. Any suggestions?

I'd keep the 2.5" exhaust. With the power that the 347 will be making, I'm sure you won't always be juicin it...so I don't think the 3" exhaust will be necessary. Last year I got rid of my 2 chamber Flowmasters that I had forever. There are MANY other aftermarket mufflers available today that are produce more power and are quiet. Last year I put a Dynomax Super Turbo cat back system on. It definitely boosted my power. I could feel it the first time I drove it even! My 1/4 mile times dropped over a tenth from this one upgrade. Plus it's much more quiet. But once you bring the rpms up or nail the throttle it roars pretty good. I definitely like the sound of my exhaust better now with the Dynomax. It sounds much more crisp & healthy. If you plan on changing the headers, I'd definitely go with long tubes. Not sure what size on the headers though. I haven't ran long tubes for probably 10 years, since I sold my '84 GT.