As far as I know, only the earlier Pinto engined Sierras are likely to used leaded petrol.
The CVH & 2 litre DOHC engines are fitted with unleaded heads.
I asked my local engine builder recently for a conversion cost, the price was just shy of £150.
His advice, " The old valve seats would be hard as hell due to the mileage " " If you are not going to be running up & down the motorways, you'll be unlikely to do the valve seats any harm "
That said, if it were my build & was going to fit an early Pinto engine, I would opt for a conversion.