71 350 running hot coolant backup up problem

[Gromit]

My '70 C10 is set up similarly and I'm not running the bypass hose from the water pump to the Edelbrock manifold either. I do get a little heat soak sometimes partly due to having an aluminum vs. phenolic riser between the carb and manifold - but I don't experience overheating of the coolant.

The first thing I would try would be to figure out the existing initial timing (after first verifying the TDC mark on the harmonic balancer is accurately reflecting true TDC)..It will still start and run good at idle even if at zero TDC.

Because with modern gas as you know if your initial timing is not sufficiently advanced it will cause exactly the symptoms you describe.

Also less important and I do understand why you are running just plain water while you sort this out but as you know plain water will boil at 212F - so maybe a little antifreeze just to raise the boiling point will be a little safer for the motor.

I noticed you have well over 4,000 posts on this forum so I'll be surprised if I'm going to be of any help to you but at least hopefully this will encourage others to chime in.