I whole heartedly agree with the finding out IF the converter is really plugged or you have another problem that is being blamed on the cat converter!
You've already got a single exhaust on the car, so a bit more restriction is not usually noticeable anyplace except WOT. One person, ages ago, put a pressure tap in the Y-pipe of his exh system. Seems the "back pressure" in normal driving was really low, but at the top of low gear in "D", THEN it got high (being it was a single exhaust car).
In about the earlier 1980s, there was such a thing marketed as a "Test Tube". Its purpose was to be installed "as a test" (which is legal to do) for clogged cat converters. Yes, many people got them installed on their cars and the "tests" were "longer term" than not. I was one of them.
A piece of the correct-diameter exhaust pipe, bent appropriately for the car, with a "fit kit" to adapt the pipe to the car. Easy to do.
As to "restrictive", when my then-new '77 Camaro 305 had a transmission malfunction at 3000 miles, as in it would not upshift except at or near 5000rpm . . . after meeting with the service manager (who said "It's under warranty" . . .), when I got back on the main county highway headed home . . . I decided to see just how "emissions choked" my 305 2bbl really was. I had to manually shift it anyway, so it was in "1" and WOT until the tach needle passed 6000rpm and was still pulling, I shifted to manual "2" and drove on. Emissions choked? Not quite! Full cat converter with a dual-outlet muffler.
On my Camaro, the test tube increased the sound of the exhaust (in the car) to levels I did not really like. Louder than suspected. I discovered that a '71-era E-body resonator (under the reaf floor pan on factory dual exhausts) was the correct size to replace "the pipe". That quietened things down very well! I later put an aftermarket converter on the car, anyway, for general principles.
I'm thinking you need a different source for your exhaust system items than the local auto supply! Using a universal converter will need the assistance of a muffler shop, all things considered. Holley sells them, among other places. Have you even looked at
www.rockauto.com to see what they have for a Formal Chrysler of any model year?
ON the other hand, all you probably need is the pipe just behind the y-pipe (and in front of the muffler) for a '74 Chrysler. IF the y-pipes and mufflers are the same for the '74 and your model year, changing that intermediate pipe would FIX your issue, seems to me. All bolt-on stuff, IF such can still be bought from Walker Exhaust or similar.
As there has been NO mention of drivability or "power" issues, the cat converter is most probably NOT the issue, even it might have some passages covered over with burned crankcase gunk. That will or should NOT cause a momentary rough idle situation. Put the car on an engine diagnostic scope and watch the spark traces for irregularities related to the ignition system. Chase THAT rabbit.
CBODY67
