is this wrong...

I did an exhaust straight off the manifolds before. While it was only a 2-1/4 pipe size, it was a mostly bone stock 360 in a '79 Dodge Magnum. The engine was getting terrible mileage. (10 mpg hwy.) and I was annoyed by it.

I ditched the stock computer with a MSD replacing it and a junk yard truck distributor. I also swapped out the 2bbl. For a 4bbl. Fed-Mog AFB 625 on top of a stock intake.
The exhaust was given a "H" pipe and Thrush turbo mufflers. At the time, I needed to have the cats on the car. So twin cats were used.

After careful tuning of the carb and distributor, w/ 2.76 gears and 235/60/15 tires, I was able to get a best of 20 MPG's on the hwy. performance picked up nicely. No cam swap done. Though should I have entertained such a thought, I would have looked at a split duration with at least 8 more degrees of duration on the exhaust side.

Have yours professionally done with the mufflers you want or look into a kit. TTI should have something that will work. You can get there kit with or with out mufflers. IIRC.

TTI also has headers with no ground clearance issues, but, they are pricey. You'll also get what you paid for with them.