I was doing this restoration on a 79 454 El Camino in early 2000’s. I removed the front section (that holds the headlights and radiator) and found battery acid rust way deep in the cracks that sandblast couldn’t reach. I poured muratic acid (swimming pool acid from Home D.) in the cracks, let it set for 5 minutes then rinsed it out with water (at a distance of course). Once dried, I poured that POR-15 BlackCoat into the cracks and that dude is still driving that hotrod around.
If you heat that POR up real good, you could inject pretty easy into those little holes in the frame.
