A couple of things to think about:
In the Mar/April 1972 issue of Antique Auto, David Chambers wrote an article about 6 Cyl, Buicks.
He cited a study by AC Spark Plug which showed that a spark plug gap of 0.040 was best. I drove the 1928 Master over 9,000 miles including 150 mile trips on the freeway and was happy with that setting.
Regarding blocking off the heat pipe to the carburetor. I did that and was not getting as good performance. I opened it up again and removed the butterfly from the exhaust manifold valve. I felt that it ran better. A side benefit is this: If the exhaust manifold valve is frozen and anyone pushes on the dash heat lever, an original heat plate can break. With the butterfly out of the valve, you can leave the linkage to the valve connected and disconnect the rod to the heat control lever behind the dash.
Adjusting the carburetor which everybody wants to mess with first and is the least likely to be the problem. Use a vacuum gauge. I found that it is smarter than I am..