There are many variables, my 84 351w has 100,000 miles I pulled the timing chain cover to replace the front seal and upon inspection the timing chain was very stretched, could almost take the chain off the gears without undoing the cam bolt, If you do replace it spend the extra couple bucks and get the double roller. They last alot longer than the single. One way to check slop is remove the dizz cap so you can see the rotor, turn the engine over by hand so the you see the rotor move, then turn it back the other way. If the rotor turns the opposite direction immediatly then it should be fine, if it has slop then farther inspection is needed.