Rat,
Very rarely does Mercury give out easy horsepower! Sometimes we share some common components, specifically in the V-6. I did a quick check with no serial numbers but the only 40 and 60 engines that share similar architecture is the 3 cyl starting around
G6----- serial numbers. Unfortunately these two engines do not use the same powerhead replacement numbers. The 40 and 50 do though, so that may be what people are refering to. If they don't have the same replacement powerhead numbers, then they more than likely have different port timing or compression. If you could post your 40 serial number I could check further, I just took a guess. Maybe newer/older ones are different.