Your O-300-D is one of the types that has a harmonically balanced crankshaft, with 5th and 6th order crank dampeners. (all O-300's have them) This gives the bottom end much longer lifespan than the earlier engine without crank dampers, and makes the crank and cases pretty indestructible. It's extremely rare to hear of a catastrophic failure in the O-300 family. Cylinders and related components are a wear item, and they are replaceable pretty easy. I recommend replacement of wear components on condition. Mike Busch is the mathematician, and piston engine guru and he has long adopted the 'on condition' replacement of engine components.
The mfg TBO is a dartboard. For almost all piston engines, the lower unit will last double the repl time of the exhaust valve. Other wear components are somewhere in between. Busch has recognized a long mathematical engineering concept called the 'bathtub curve of failure'. Meaning, a component in an engine has a higher rate of failure in the first 50 hours, and concomitantly a higher rate of failure near end of life(for the specific component, crankshaft, exh valve, oil pump, etc). Meaning that once the component reaches around 50 hours, it's highly likely it will survive through the bottom of the bathtub curve and reach it's nominal end of life.
Time between overhauls (TBO) is a strange concept. The FAA requires aircraft engine manufacturers to publish TBOs for their engines, but doesn’t require aircraft owners to abide by them.
www.aopa.org
So, based on the math and statistics, your 'new' jugs are a more likely place of failure than catastrophic failure of the lower end. The O-200/300 family do not have oil filters. The best investment you can make by far is an external filter which will trap metal media, and extend oil changes. I have them on each of my legacy Conti engines and I check the filter media each oil change. Oil analysis is also a great tool to verify component gradual and rapid failure.
Summary - go forth and fly past TBO. Put on a filter setup. Do oil analysis at each oil change and make a chart of metals in each report.