1989 Oldsmobile 88 it started with a very faint ticking

I noticed a very faint ticking in the morning and by the end of the day it was a knocking. It was not the oil and had pleanty of coolant. Then I was driving and smoke came in the cab and all the fluids leaked out. I knew that the heater coil had gone bad. So I bypassed the heater coil and at the same time I went ahead and took the serpentine belt off. That showed me that I didnt have a blown head because the knocking stopped. Then I hooked it back up and the knocking came back. I figure its probablly the water pump. It seems to be running ok except for the knocking and no heat of course. I am wanting to replace one part and not go down the chain. What is your opinion. And how difficult is it to change out that damn heater core