Shifting problem with 833

I havn't had my 67 Barracuda long and was wondering how to check the oil in my 4-speed (I think it is a 833) transmission. I have always had automaticis and don't really know. I guess it has a plug like the rear axle which when it is full it is close to the top or level with the top of the plug opening. I would like to change the oil to some good quailty oil like mentioned above. So how do I check it?