In 74-75 the factory used an HEI with an external coil mounted on the side cover under the AC. That was also the first years for the HEI. I've also read that you can't convert the later HEI to an external coil because of the indexing on the cap, don't know how true that is. You could install a Pertronix in your original distributor for HEI. Hope that helps.
Larry
