Before you tear it apart, here is something to try- The older Chevy small block V8 rocker arms could be tightened down from under the valve cover. If yours are similar, perhaps you could just tighten the rocker that is worked by the faulty lifter a turn or two and reduce the tapping noise.
If you do decide to tear it down:
I wouldn't risk running the engine with coolant contaminated oil. I have heard that the coolant can trash the bearings.
Here's an idea to eliminate the coolant/bearing issue:
Drain the coolant and replace with plain water. Run the engine.
Drain and replace with plain water. Run the engine.
Drain and then pull the head. If anything enters the crankcase, it will be plain water- which will not hurt the bearings and will boil off completely when the engine gets hot after the job.
The other issue is small chips of head gasket material that fall down into the oil return passages as you scrape the block clean- changing the oil helps get rid of these. Maybe you could just flush the passages out with a funnel and a quart of cheap oil after you finish scraping (leave the drain plug open). Stuffing rags in the holes while you are scraping will also help minimize this.
This website mentions "plastic lifter guides"
http://www.enginebuildermag.com/Arti...2l_engine.aspx That could also be part of the problem.
Expect to spend at least 1-2 hours cleaning off the old head gasket material. Just don't gouge the head or block surfaces (which is easy to do if either is aluminum). Don't be tempted to use the abrasive disks (that you put on the end of a drill) to remove head gasket material if the surface is aluminum. You will remove aluminum from the surface and make the new head gasket less likely to seal (I found this out the hard way).