The only externally balanced engine I can think of off the top of my head is a 400 small block. Did one of these vehicles have a 400 small block in it? If so, that flywheel WILL NOT work with an internally balanced engine such as the 305, 350, 327, 307, 327, etc;
As mentioned above, there are two flywheel tooth counts 153 or 168. Match the starter to the flywheel and your problem will be solved. If you have a 153 tooth flywheel. Look for a starter from a 1970 Olds product with a 455. It will be a high torque starter designed for the 153 tooth flywheel and will bolt directly to your small block.
Or, go with one of the gear reduction starters off like a 95 Chevy 1/2 ton pick up with a 350. That starter will be VERY light, will pack one hell of an electrical punch, and will start an engine even if the battery is weak as it draws less amperage than the old style starters. It is also designed to work with a 153 tooth flywheel.
If you need a 168 tooth starter, buy one for a '69 Camaro with a 350.