Regarding the ignition system checkout chris, we need more info as to "which" ignition system you are dealing with! There are many possibilities and since the po virus infected this rig, there is no telling what system you are dealing with!
And just because the parts lists say a certain system was used, means nothing at this point in time, very few of these rigs are still "original". And the IH service manuals are full of mistakes, omissions, and discrepancies in this regard also. That information is more than 30 years old and the print versions were never actually "updated", that was done for the dealership by issuing replacement loose-leaf pages and "bulletins". Sometimes ....some of us have access to "some" of that info simply because we collect this kinda stuff, but there is no way to obtain a full set of "updates" unless the folks that control the publication of that data decide to make it available for sale.
My suggestion at this point is to review all the threads in the ignition forum here, you will learn how to identify what ya have, and how to diagnose/service also! And that forum is updated just as often as the others! In fact, I'll be throwing some new stuff up there today once I get all the pics lined out.
You will see that many folks "thought" they had carb issues, when in actuality the root problem was ignition-related. Thus making certain the ignition system is up to speed is imperative before touching the fuel system!
"base" ignition timing for a well-performing 304/345 running "regular" grade fuel will be between 6 and 10 degrees btdc at idle if the distributor is in good shape, some can tolerate as much as 12 degrees advance in base timing. That holds true for either a breaker point system or an electronic trigger system. That is not a "specification" from a service reference. "specs" were for verifying that the rig would meet "emissions" numbers when properly serviced, but do not necessarily mean that drivability was satisfactory! This is a very complicated issue for folks not trained in this type stuff.
A 392 motor can normally tolerate only 5>6 degrees of base timing on regular fuel.