The extension ring sits on top of the base wedged into the inside of the curve, so the dizzy cut out has nothing to do with the ring.
I assume this is after market and not original to your vehicle as the aftermarket bases and rings seem to need some persuasion with a rubber mallet according to other threads here and on the web.
Seems the best solution was to place the naked ring (rubber seal removed) on a soft flat surface, then place the base on top of the ring then take a 2x4 across the bottom of the base and then whack that gently with the rubber mallet and move across the base in small increments until the ring is evenly seated. Then flip unit over and mount your rubber seal and install.
Original equipment slid in perfect and you can at any time remove the ring with little effort.
I found that I had to slightly re-form the area near the distributor. I slightly rolled the edge of the ring to give a better fit using a body hammer and dolly. Basically the taper on the ring was interfering with the taper on the base and forcing the ring back out. I had the same issue with it popping out but once I reworked the lip, the ring stays in and I can remove and replace it easily. Also check the cutout for your carb. My air cleaner was poorly formed there as well and I had to roll that edge for it to properly seat on the carb.
The "rubber mallet" technique that 69 Pace mentioned what basically what the tech support person at Classic Headquarters told me, and I am sure many others, to use to get these parts to seat. Seems that they are well aware of the issue but I am guessing don't want to spend the money to re-tool the parts. Good luck.
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