I have pulled valve covers at ~100K mi on Slant Six, 273, and 383 engines. Mine always look fairly clean. Yours look like what I have seen in junkyard engines. I suspect one has to skip oil change or drive hard and hot to get sludge like that. Still, if you have good compression, the engine is fine. Perhaps you can clean w/ kerosine with the pan off, then run chemicals thru the water to clean the passages. I am guessing you have a 318 since the alum maninfold went on easy ('66 273's have different angle holes).