66 was the first year the 440 block was cast , nothing wrong with them , the main difference is the extra rib cast on the outside of the block @ head bolt depth to prevent cracking ..... when was the last time you saw one crack there , personally never . I used a 66 block for a race engine making over 600 hp gross & running mid 11s in a duster weighing 3440 with me in , I shifted it at 7000 rpm for 9 years , no issues at all .