It depends on the engine. Ford has used both stamped steel and cast iron for rocker arms in the 300. The cast iron parts are "rail" type, which means they have rails at the valve end that locate the rocker on the valve. These work OK in a stock engine, but are known to cause premature valve guide wear because of the side loads they put on the valve. The stamped steel parts were used as pedestal mounted rockers on the EFI 300, and were used as stud mount on most early Ford engines before they went to the rail type. With stock lift and valvespring forces any of them are obviously OK for many miles, but when the valves are opened further and the valvetrain stress increases they get less and less reliable.

Mileage on a roller rocker is a liability. Most are on their way out by 25k-30k miles. However, I have one customer with a 96 Impala that is extensively modified including GM's LT4 Hot Cam that measures 224@0.050 and 0.525 lift with GM 1.6 ratio roller rockers. I had his engine apart a few months ago to repair a valveguide and the rockers were all still in perfect condition. The engine had 173k miles on it at that time, and probably 60k miles on the rockers. Most of the higher priced rockers will last longer than the cheapos, and the stainless steel ones will last longer than aluminum. When the valvesprings get stiff and the lift increases they often last longer than a stock rocker, but that probably won't be an issue on a 300.