Todd: Nice radio! I've never worked on that particular model, but most of the early Zeniths had a dial glass gasket that goes around the edge of the dial glass. These are often dried out, cracked, and sometimes stuck to the glass and frame. If you have enough clearance, you might try running an exacto-knife around the edge of the glass to free it from the old gasket material. Then VERY GENTLY, see if you can pry the glass up without putting any stress on the glass. It should come loose. Radio Daze sells replacement rubber extrusions for dial glass in various sizes. Great Northern also has it.
Steve
Todd: It occurred to me that you might be having trouble with the spring wire that holds the dial glass in the mounting frame. If yours has that, use a pair of needle-nose vice-grip pliers to firmly grab one of the free ends of the spring wire. Carefully lift one end of the spring wire up and out to clear the nearest tab that holds it in place. The spring can then be removed, and the dial glass should come out easily. Reverse the process after you've replaced the dial glass gasket. If you need any pictures, I can post one showing the spring wire used on a Zenith 5-S-29 dial.
Steve