For what it's worth i have some cheapo summit headers on my barracuda. Lowered the front end pretty decently, summit adjustable shocks on all four corners...the only time i've smacked my headers is going about 45 through an intersection i didnt know had a huge dip in the middle of it. Under normal driving, besides that, they were kept dent free.
I would run 3" to 2.5" smooth reducers to mandrel bent 2.5" aluminized pipe, an X-pipe cross-over, dynomax, hooker, spintech mufflers are all good. I have flowmasters on my car, i wouldnt recommend them. Exiting exhaust like a AAR Cuda or a T/A Challenger will help promote scavange of the exhaust at speed, otherwise just ran out the back of the car will do. I would add atleast one oxygen sensor bung to the exhaust if you plan to get serious on tuning/tweaking that 340.
