I probably wouldn't worry about it that much. Oil and water make a creamy tan mess when mixed. If the antifeeze is clean and the oil is clean I would move on to common engine cks such as does it miss and do a compression test. This of course is based on asking price. Is he tring to sell the truck complete or just the engine? I would also ck the stroke to make sure a 360 hadn't been swapped in. To me it is about cost and risk at a hundred my risk is lower then about a thousand. I have a 360 out of a small bus van 100 bucks (may build a big stroker) as well as a clean 390 from a mid 60's pu $500 both included c6's.
NCbird on the Coast of NC
"Dads Bird" for my father