questions about timing

[frankym]

I had some issues around timing and welcome any suggestions. I recently replaced my exhaust header gaskets because I was convinced that the sound I was hearing under load was an exhaust leak. I discovered that the sound I was hearing was engine knock. I took the car to a couple of shops and the first shop adjusted the timing a bit by seemingly randomly turning the disributor without a light or any other guideposts. The knock continued and I then took it to another shop that had more familiarity with old school carburetor motors. He found that the total timing advance was set at around 55 degrees. He adjusted the car for a total advance of around 39 degrees. The mechanic said he likes to tune cars for a 3,000 RPM total advance of around 36 - 40 degrees. The knock dissapated a bit but I marked a line on the base of the distributor and turned it clockwise (at the mechanics suggestion) to retard the timing just a hair. The ping/knock was still apparent and the only noticible difference is that the car now cranks over a little more slowly when it's hot. Ironically, this is the first time the cranking has been slow when the motor is hot (suggesting that the previous setting was not so advanced after all).
My question is, what is the appropriate range of total advance for a crate 351 windosr with cast iron heads and if I wanted to retard just a little, should I be turning clockwise or couterclockwise?

Any help would be most appreciated.

Thanks
FM