wow... ouch.. OK Ill bite... the 360 head has a much larger cumbustion chamber. Larger cumbustion chamber = less compression. your stock 318 probably had 8.5:1, after putting on 360 heads youll probably have closer to 7.5:1.
Casting #302 heads from the mid=late 80s Chrysler cop cars & taxi cabs with 318s have the small chambers but also have same valve size as 260