1971 Mercury Motor, can I use 1981 powerhead?

[bowhunter_58]

I have a 1971 Mercury 40 HP motor and justed purchased a 1981 Mercury 40 HP without the lower unit. I am not having any luck finding a lower unit for the 1981 and was wonder if I could just use the powerhead from it. I have located several 1977 and 1978 lower units but was told they will not work on the 1981 motor. Please help....

Thanks in advance.