Not knowing what yours looks like, I'm going to add to Tim's post anyway. Unless your engine has a special distributor, all 216/235/261 engines have the same set up as shown in Tim's attachment.
What is not shown clearly in Tim's attachment is the distributor clamp. It can hide in plain sight if you don't know where to look...and it is not easy to work on. The bolt and nut of the clamp can get covered in grease and be hard to find if you don't know what you are looking for.
In Figure 32 of Tim's attachment, see the square nut that is just above and to the right of the octane selector? That is the clamping nut for the distributor clamp. The clamp itself is the band that goes around the distributor shaft...that square nut is a part of the assembly. Not shown is a view from the other side of the distributor. On that side, there is a slotted head screw that attaches to the square nut. To loosen the distributor, loosen that slotted screw. If your assembly is covered with grease, consider squirting it with brake or carb cleaner so that you can see what you are working with.
I use a slotted head screwdriver bit in a socket wrench to turn that screw because only a stubby screwdriver will fit into the available space and that does not always allow enough torque to loosen or tighten the screw, at least with the rheumatism in my hands.
Many people set the timing closely using the distributor clamp, then fine tune it by using the octane selector. I find I can fine tune it using the vacuum gauge technique with just the distributor clamp as long as I am using the screwdriver bit in a socket wrench.
The indicator line for the octane selector that shows up so clearly in Tim's attachment can also be hard to find. It is etched into the flat part of the block itself. Cleaning the area well will allow it to miraculously appear. Once you find it, consider marking it with some paint. Knowing where it is will allow you to set the octane selector at zero before you begin timing.
Matt
