Over heating straight 8's has got to be one of the top 10 (or higher) topics - very common and as noted has very many culprits to work through.
This was my input on another "overheating"
post:
>> After replacing the water pump with a rebuilt one, having the radiator checked and cleaned, setting the timing "by the book" (6 degrees past TDC), checked for good-strong firing from the distributor, pulled and cleaned the distribution tube and cavity, removed the freeze plugs and pressure washed the inside the cooling jacket best I could, replaced the timing chain (while I was there), none of which resolved the problem completely. It still ran hot but didn't boil over. Temp gage would get up to "H" and stay there, and only slightly come down with higher RPMs.
The remedy I finally discovered was to set the timing "by ear" - which ended up (with my engine) 12 degrees. Seems strange, but it starts fine and runs and idles at normal temp. <<
Good luck - and enjoy resolving the problem. It will be very rewarding when you get it - I promise!
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