Rough idle turned to crank then ka-thunk

Slanty's have timing chains. They actually use the same chain and crank sprocket as the big blocks (cam sprocket is different). The factory cam sprocket had nylon coated teeth. Most aftermarket replacement sprockets were not nylon. The heavy duty slants in some model trucks had double roller timing sets from the factory. If the nylon teeth on the cam sprocket becomes excessively worn, the chain can jump and change the cam and ignition timing.
Also as stated the nylon distributer gear can break, also.
PS: Just a FYI. If the timing chain breaks or jumps on a 225 engine, not likely to cause any other damage. However if the chain breaks or jumps on a 170 engine serious valve damage can occur. The 170 is an "interference" engine, whereas the 225 engine is not.