If it's a real street car, I would stay with 2.5" mandrel bent pipes. Yeah, you would be giving up horsepower and not an insignificant sum of it. With a 3" exhaust, however, you are going to have to really consider packaging constraints and noise. A 3" exhaust, all things being equal, will be significantly louder.

My '65 Fury has 1 7/8's tti headers, a full 2.5" tti exhaust out the back and the longest body of muffler that Borla makes. When I ran that exhaust on a mild 383 it was nice and mellow. After I moved to my 512 the car is now noticeably louder, with an emphasis on low frequencies. If I had 3" pipes right now I would definitely be looking for ways to quell the decibels. Additionally, there is no way in hell I'd run a larger header on my car. The current pipes are big enough for an afternoon of head scratching and vocabulary building as it is.

And for reference I use a 512 with a Molnar crank and rods, Diamond pistons (stock PN), and Trick Flow heads. It makes around 10.3:1 and uses a Straub solid roller cam.