Hi, This past week, my 351-C suddenly overheated (apparently the
thermostat stuck) and now when the engine is warm and running above
idle, there's a thudding/knocking sound which appears to be coming
from somewhere down in the block around the number 8 cylinder.  When
the engine is cool, or at idle, the sound goes away.  Oil pressure is
near 80-lbs at idle, and drops back to around 60-lbs when the RPMs
increase, so it appears that there's no oil pressure problem.

Note: the engine was running at normal temperature while driving, but
the coil went bad, causing the car to stop, and when it stopped, the
gauge went over to hot.  Enough water in the radiator, but was muddy
looking, like coffee, and some thick muddy gunk inside the radiator
cap; have since flushed the radiator... looked ok a few months ago.
Note: I haven't used any cooling system additives, just water and
antifreeze; radiatior was new a few years ago.

When a mechanic dropped the oil pan, there were no signs of any metal
particles.  From what I could see of the piston skirts, there was no
obvious scarring.  The mechanic checked the rod bearings for any signs
of wear and didn't find any; said that he couldn't find anything wrong
without tearing the engine down further and didn't think the mains
bearings were the problem.  He didn't say if he used anything to check
the actual gaps; I'll have to wait until next week to find out.

Also, the timing was a bit off for a while with some occasional
detonation due to a distributor that wasn't curved right, but I didn't
think that there was enough detonation, lasting for a long enough time
to do any damage (hopefully not, anyway).  The mechanic also suggested
that the tops of the pistons (hypereutectic) could be damaged or have
some carbon buildup (the plugs look ok, no significant carbon buildup)
causing the knocking (not detonation pinging) sound?  That doesn't
sound right to me.  Any thoughts?  4V heads with small quench
chambers, and no smashed spark plugs; 10:1 compression.

The engine was rebuilt back in 2001 and not a lot of mileage on it
since then; it also sat unused for most of the past year, if that makes
any difference.

Any thoughts on anything that could be causing this, which can be
repaired without the engine having to be pulled and torn down?  Am I
correct to think that doing any driving with this engine is a bad idea
until the problem is corrected?  I don't relish the thought of another
rod through the block, and a disintegrated piston, like I had a few
years ago after overheating in stop/start traffic.  

Are there any safe oil additives to use in such a situation?  Never
really trusted using engine-oil additives, don't know much about them.