If the heads are pre-1972, they will not have hard seats. If you don't use hard seats with unleaded gas, you'll experience a problem with recession, a condition whereby the valves will sink down into the heads as they eat away material at the seat.

You can either have the machine shop install hard seats and valves or find a set of heads from a 1972-up motor that would have been designed to run on unleaded.

Some fellows will go ahead and run unleaded on early heads, sometimes using a fuel additive, but I personally prefer to do it right while the motor is down.