Pletch,
I have the shallow distinction of having a non-running 228 in my GMC 52 pick up.
If you use Tim's link to the GMC website and click on the links, you will find that info for the 228 is also included, even though they say "248-270".
So, from the site..
Dwell/Point Gap specs .016-.021" 38-45 degrees
Initial Ignition timing point is UDC (upper dead center) for the 228 and 248 and
5 deg BUDC for the 270. I believe the mark reads "UC" on the flywheel.
I believe all readings are with the vacuum hooked up.
From p 173 from the GMC site, under ignition system it shows maximum centrifugal advance for the distributor is 10-12 deg. at 1475 rpm and then a footnote that states these are one half of the engine degrees and engine rpm. (Maximum advance in engine degrees is 20 deg.) Perhaps this answers your last question.
Hope this helps, you'll have to excuse me now it is 60 degrees outside on this 5th day of Jan. in Kansas. Too rare to not enjoy.
