I am working on a 57 Sedan de Ville with a 365 in it. When it came to me, I did the normal things to get it running, check for compression, fuel, and ignition. I got it running and it ran pretty well. Ididn't run it very long as the cooling system was drained. While cleaning up, the gasket scraper I was using slipped and hit a head bolt and it MOVED! On further investigation I found broken head bolts on both sides. I removed both heads and took them to.my local, old school, machine shop. Both heads are cracked in the same location. The third cylinder back (#6 & #7), the crack curves between the valves. Now to my question. I understand that the 57 castings (1465406 & 1465431) are prone to cracking, and are difficult to find. What heads should I be looking for to use as replacements? I have seen the 58 - 62 heads and the 63 390 heads available but I need to know what is simplest to use as replacements.