Well, yeah, a plugged up exhaust will make it pop back. The hot exhaust is still in the cylinder when the intake valve opens, and boom!
Not necessarily the converter, some Fords use a two-ply exhaust pipe and the inner ply can fold over inside the outer one. That'll drive you crazy trying to find it! I used an infrared thermometer to find the hot spot. If you don't have one just look to see if the pipe is discolouring somewhere along its length with no apparent reason. It'll be cooler from the restriction onward.
On my old Marquis the cut-off pipe was three-ply in one half of the circle, and one-ply on the other, looked like it was made that way! Downstream a foot, it was normal. No flow whatever! All the gases were going through the intake heat riser port, and out the other pipe.