Big block exhaust is still loud

Imo, the secret to a quiet good flowing exhaust, is getting the proper size tubing, and then getting the biggest longest 3-pass muffler with that size tubing that will fit under the car.
In your case, since you already changed the mufflers without using the biggest that would fit, I would add the longest 3" glasspack I could find that will fit in the tailpipe alongside the gas tank.
Edit: did you use the triple pass muffler, or the straight thru?