How to Replace heater core 81 ford f100 pickup?

I replaced my heater core in 1991 for my Ford F100 I6 300. If I remember correctly , it was a tiresome job because you have to remove the black plastic housing under the hood; which has a lot of small nuts. If my memory serves me, the heater core is under the hood, inside the black plastic compartment. The core is immediately under the place where your heater core hoses route to the firewall. You should have an easy time finding it. good luck