New car, new problems; 76 Duster 318

"I have a feeling the Cat may be plugged. I ran it today after putting a new air filter on (haven't had time to do any more to it), and noticed the exhaust is shooting black soot... Not smoke, soot."

Theres the reason for the miss. (Way rich for some reason)
I suspected something like that when you said the miss was worse after you drove it for awhile.
These cars don't have a cat.