Jerry supplied info to poster about GM 2.8 Dizzy. You asked what years and where to get. You can get a donor from a S10 or buy a new one. I searched the facts about S10 and the vacuum advance style for 2.8 and posted what I found and a source. Don't know about the Langdon. Will check. I have never done the swap and only know what Jerry and others advise. Was just trying to help with years and source and history of 2.8 to add what I could to the posterity of the post for future searchers.
To my untrained eyes, the Langdon looks like the O'reilly's. Same dizzy, same cap. Already modified, don't know what modification is necessary. I think there is a TIP here on it.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn