351-C with deep knocking sound

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.
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Your guage is wrong, idle will never be at 80 psi. 6-8 at curb idle hot and 40 -60 @ 2000RPM hot. That is for stock, a high flow pump will be a few pounds higher.

So he didn't inspect the mains? Looking at the bearing shells, unless there is obvious wear means nothing. The bearing clearances should be measured.

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