Engine: 1969 350 4bbl (LM1)
Carb: Quadrajet 7029202
Distributor: Original. Points type.

Rebuilding the distributor and bought a replacement vacuum advance canister. Looking at the original there is a rubber-like bushing on the end of the rod that looks as though it may limit forward and backward travel of the rod. The bushing sits on the end of the rod that curves up 90 degrees and fits into the bottom of the advance plate that holds the points.

Can't find a replacement vacuum can anywhere that comes with this bushing, and can't find a replacement for the bushing itself anywhere either. How critical is this rubber bushing, is it even necessary if replacements don't use them?

...Looking at the original there is a rubber-like bushing on the end of the rod...
Can't find a replacement vacuum can anywhere that comes with this bushing, and can't find a replacement for the bushing itself anywhere either. How critical is this rubber bushing, is it even necessary if replacements don't use them?

The little bushing is actually part of the advance plate assembly in the distributor. GM provided them in the replacement plates sold over the counter.
It may have stuck to the vacuum canister rod and came out when you removed it.
They have used both rubber and a plastic bushing over the years.
I don't know of a source for them offhand, but I would try an Auto Eletric specialty business first - if they don't have one they probably have some laying around from old distrbutors.
One of the many specialty engien electric service guys that advertise in Hemmings would also probably be able to get you one.
If it's not installed, the worst case would be the timing would vary by a small amount constantly as the plate moves back and forth. There's usually quite a bit of friction working on the plate, due to the felt bushing and retainer, so I'd dought it would bounce around during cruising.

So what you're saying then is the bushing is meant to keep the breaker plate from moving around on the vacuum advance can shaft (up or down or back and forth)? If that's all it does, would a simple piece of rubber hose or nylon tube cut to size work just as well? I didn't know if it was supposed to control movement of the breaker plate on the rod, or if it was supposed to limit the back and forth travel of the rod in the slot of the vacuum can frame.

...would a simple piece of rubber hose or nylon tube cut to size work just as well?...

Well it's really only there to isolate the two. I don't think it really keeps either from vibrating, but neither of these two items probably vibrates very much unless something is worn or wrong. The vacumm advance canister is fairly slow acting on it's own and the plate may be prone to a little sloppiness if the lower felt washer is dry or worn, but not to much.

Yup, I've cut small pieces of tubing or plastic rod to replace missing bushings on several different distributor types when these thingys are missing. Just finding something that will take up the extra space and live a fairly long life in there is the key.

Well, I have some nylon tube here exatly the same I.D. and O.D. as the old rotted bushing. It's good to 250 degrees F. so it should live well in there for a long time. I'll keep you posted how it works.