BAD lean spot.... any suggestions??

Hi Y'all, I'm new here and I am trying to make my old slant 6 run better.
Vehicle: 1984 Dodge D100 Pickup with 225 /6.
Totally stock to include leanburn ECU hanging on air cleaner.
A833 4 speed overdrive manual transmission
3.55:1 open diff rear.

Anyway, my problem is this, a SEVERE lean spot, bucking and missing under load from about 1000-1500 RPM's. I have checked and adjusted the timing, replaced plugs, wires, distributor cap, rotor button, carburetor with a new Holley 1945, and I am still at a loss. I have replaced all vacuum lines, and have found no vacuum leaks. I am registering about 22 inches of mercury vacuum at idle and is smooth with a 750 rpm idle. The truck seems to accelerate better when cold and still "on choke". At this point I am suspecting a clogged catalytic converter since my dash mounted vacuum gauge seems to be showing only about 5 to 10" of vacuum at highway speed. I am getting a new catalytic converter next week and will see if that corrects my problem. I suspect the cat because of vacuum readings, the rattling of it, and the fact that the truck ran MUCH better earlier this week when I was on a bumpy road and I presume the honeycomb rattling around shifted and allowed "better" flow. When this occured the truck had smooth acceleration in my 1000-1500 rpm trouble zone. Yes, the accelerator pump is functioning fine, I have tested that too. Any suggestions??
Thanks!!