First of when checking for total advance, make sure that the vacuum hose to the vacuum advance is disconnected and plugged. The term "total advance" can be a little misleading, causing folks to think - well - total
must mean everything connected with all the advance you can get. It doesn't. It really means total mechanical
advance with the vacuum out of the picture - just like your engine is going to see at wide open throttle under load. Your engine is not making any vacuum at that point, so even with it hooked up the vacuum advance isn't going to be adding anything to the picture. It's not really possible to duplicate those same conditions with the car sitting in your driveway in neutral
-- so to get an accurate reading you have to take the vacuum out of the equation by disconnecting it. Under those conditions, you want about 36 degrees total by about 2500 rpm "or so".
But yeah, like GeeTee said, the factory spec for initial timing in these cars is 6 degrees btdc. If after setting it there you still have too much, then the way to take some out is just like you said. Put a bushing (or a larger one) on that pin that limits how much travel is possible in the mechanical advance mechanism. The old timey distributor hop up kits we used to get that came with various different weights and springs usually also included a few bushings of different sizes just for that purpose.
You might be able to get it done with the distributor in the car, but it's a lot easier to do if you just go ahead and pull it, drive out the roll pin that holds on the drive gear, and then slide shaft and all up out of the distributor body. Before you remove it from the engine, pull the cap and bump the starter over until the tip of the rotor is pointing at something you can use for a reference point, so you'll be able to reinstall it in the same position when you're done. It may take some finagling to get it to seat all the way down with the oil pump drive tang engaged because the bevel cut gears tend to move the pump a little when you pull the distributor out, so don't be surprised if you have to install it a few times to 'walk' things around back into alignment.