"they don't always break, I'm sure some people's last 300,000 miles. Wouldn't it have probably been replaced if it has lasted this far?
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You are playing with fire on this timing chain issue. Most chains don't break but related pieces in the timing chain system, guides and rails, get weak and brittle with age and it is these pieces that break and jam up the timing chain in the timing gear teeth causing the chain to jump and that leads to major engine repairs (valves and pistons). The general rule on this forum for timing chain and related item replacement is 10 years/100,000 miles, whichever comes first. Anything past those limits and you are rolling the dice....

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1986 560 SEL (159K miles)