I did mine several years ago after letting it go to long. The anitfreeze leaked into the oil and caused a good deal of sludge to form. So, if that is your case, I would recommend replacing the push rods and roller bearings too.
I spent some time cleaning the innards as best I could too.
I also use a quart of Risolene with every oil change to break down the remaining sludge.
The gasket was a big job for a me, but I got through it.