There is a better way, less destructive on the mufflers...
behind the headers rig an electrical conduit box in-line between header & muffler. Preferably near to the header. The conduit box is the pipe, or semi-round kind, that has one flat side-plate that can be opened. This side opening plate is held shut with two screws.
Shorten one of the screws, so that when inserted it just barely bites, the less thread, the better, leaving just enough to hold the pressure under normal conditions. Leave the other screw less than fully tight.
Floor it, running through the gears, maybe to third, pulling a grade. When reaching the top of the hill, let off the gas suddenly--- ka-POW! Blows the headers wide open, dumping through the now open conduit box. Loud as all git-out...
Add quarts of Coors, a few doobies, then go sliding sideways through intersections.
Texas. Mid '70's. How did we live through it?
Gratefully, with great vigor.
Add the driver turning off the headlights and doing the Fosbury Flop into the back seat while the passengers flop around and that was me.