What year? What style of switch you got? 69 up through SOME 78s, they had the little tab on the end with no screw to hold the ignition switch in. Later models have the screw hold down. Remove the screw and slide out the ignition switch.
Depending on which style you have, you don't have many options. 78 had a 1st design that used the tab at the end to hold the ignition switch in. There's no screw holding it in. The 78 2nd design had the screw. There's been reports from GM literature that some 78 that should have 2nd design has the 1st design installed for some reason. Go figure.
Options:
1) Rebuild what you have. PITA if you don't have new tumblers and junk, so probably not.
2) Buy a new ignition switch WITH KEYS. I like this option. It's the same amount of work to get the old one out as option 1, and to put it back in, but you'll do it in seconds and have your ignition working great in no time.
Brigg & Stratton, now Strattec, 608438, or 607872, or GM p/n 7830380 fits those ones with the screw hold down. Most 78s - up. Latest number is 19356477. Issue with these is that you get the key for that particular key slot for the YEAR THE LOCK WAS MANUFACTURED. So if it was manufactured in say, 1982, you got a J key. If 1983, you got an A key, and so forth. So if you have a particular ignition key you want to use, you got to seek it out.
If you DO have the tab and want to go with that, it's Strattec 607893. Several GM numbers, 20024673 is one of them. 20071252 is the latest.
Here's one with a J key. Ignition on our cars is the only one with the square key (supposed to be anyway, but haven't researched wagons, so they could be different).
https://www.ebay.com/itm/1560591807...d=link&campid=5335822911&toolid=20001&mkevt=1