I've been waiting to say that, doc bob. One of the strange books I've read was on how to maintain marine Diesel engines, and it had nearly a full chapter on 'pinging'.
The thing is that you can set your mixture to the point where you can't hear a ping, but the engine is still pinging. (Diesel engines, so the ignition system is not a factor.) Naturally, most of the things that apply to stopping pinging completely in a marine Diesel don't apply to a Chevrolet 235, but the point is that it is hard to detect a 'just barely pinging' engine. Better to be safe than replace a $1200 piston (many marine Diesel pistons cost a lot more than that.), so retard the timing at least five degrees after you stop hearing the ping.
BTW, pinging occurs when combustion starts somewhere else in the combustion chamber than where it should, and before combustion SHOULD occur. The piston may well still be moving towards TDC, rather than away from it. I think you can see that any significant combustion pressure in a cylinder before TDC is going to put a strain on a lot of parts. The crankshaft can't just start going backwards, so you have a case of irresistable force and (kind of) immovable object. Not good for pistons, rods, or crankshafts. Also not good for fuel consumption because the engine is actually fighting itself to provide power.
As to the idea of replacing part of a vacuum advance line with rubber tubing, be careful. My parents '64 Chevelle had a 230 in it and the carburetor end fitting eventually ended up stripped. I replaced it with a nipple and a piece of 'HiTemp' tubing. It started running like doodoo. So I got under the hood, with the engine warmed up, and tried to find the problem. Not too hard to spot. When I revved the engine (increasing the manifold vacuum) I could see the tubing collapse along most of it's length.
Basically, if you do this, be sure that the tubing you use is really suitable for the environment.
