Just buy a brand new HEI for a 1975 and up Nova 250. A universal harness supports HEI. CARDONE 841692 seems to be a good one. Probably the only way to get an actual OEM Delco is from a donor. Ebay etc.
OEM is a loose word. These parts are obsolete, so you can't really buy a new one. Just mostly copies. If you get a used one with not much axial or radial play and good teeth, you can build it as said.
The Cardone is usually a good company but the one I mentions is out of everywhere. Try NAPA. Anyway it seems that just a year later the motor got HEI's. So that is your direction. If you buy new, buy from a well known place near you and try to get a warranty.

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