Most Y blocks will run fine at 12 degrees. The problem is the dampener ring that has the timing marks isn't always accurate as it can slip after years of use, a very common problem. Also check the timing at higher RPM with the vacuum advance connected to see if the advance is working. Did you have the vacuum advance disconnected when you checked the initial timing?

Interesting point you made Steve, about the fan flattening out up hills under load. Would never apply to my car and besides that we don't have any mountains here.
I've not heard of a factory flex fan letting go but I can imagine it is possible with metal fatigue. The fan on my car has a second piece behind the flex blade to spread the load so that it is not all on the rivets. I don't know if they are still sold but I remember early aftermarket fans were very basic and the entire load would have been directly on the rivets.

Good question! I'll make a point to check that for ya.

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So I got to cleaning my timing marks a bit. CLR is amazing stuff man. I'm prepping to do a timing check/adjustment soon, just ran out today and got me a Bosch timing light. Yes, I have never owned a timing light till now. The last time we did it was with my buddy Harald and he had a crappy Harbor Freight timing light you could barely see. Anyway, his mark it still on there actually and yeah if I remember right from seeing it. The guy that rebuilt my carburetor said the timing had to be set at 12o before TDC no matter what any car's factory manual says so that's what we did. I looked online tonight because I forgot and it looks like factory calls for 6o before TDC.

I gave the guy at Carburetors and More an email to double check what he said, I'll let you know when I get a response.

core plugs ,i use the rubber ones,as it is no easy task to get the core plugs,just have to loosen the state and let it hang down.  do not loose them into the block,drive them in on 1 side then grab them with small vise grips. clean the holes the ruber expandebels work fine for me

1/2 Dude - how does the vacuum look when you are up at the "overheat" situation?

Flex fans have a problem, especially with truck applications - where you are climbing a long grade ( I was living in the mountains at the time) at elevated rpm (lower gear). Fan flattens out - moves less air at just the wrong time......... lots of load lots of revs and reduced airflow.
Conventional six blade, direct drive is a good choice - the one I have on the car now is an aluminum 7 blade off an older Pontiac. I think Ford was using a five blade on the air-conditioned cars of that era.

 
 Steve Metzger       Tucson, Arizona

I remember that flex fans were prone for the blades flying off. One customer of the gas station I worked at had a nasty slice in his hood from a blade that flew off. The dealer replaced it with a clutch fan. They were deemed dangerous and we were told not to stand next to the fan with the engine running.

Found this one for a 66' Mustang offered at O'Rileys... is this what you got?

Man that fan really does look like it would move a lot of air Ian57!

So if I wanted to check the block for blockages inside the coolant channels, how hard are the freeze-plugs to pull out and get in there? Are they difficult to get back in again? And do you run the risk of causing leaks from those things if you mess with them?

It sounds to me like the engine has a vacuum leak  - that gets bigger AFTER a period of running. Is this engine still running a factory initial time setting - with a Load-O-Matic distributor? 
Lose vacuum - Idle speeds up - Spark retards.

 
 Steve Metzger       Tucson, Arizona