You can add landed cost in the Purchase Receipt as well. You have to add it in Taxes and Charges section and make sure to select ‘Total and Valuation’ or ‘Valuation’ as indicated here:
All the past PR, Can I set it and made a total recalculation for all product and PR?
Because currently, all were ‘total’ selected and not ‘Total and Valuation’
How many would they be? For a very large number, this solution may get unfeasible. Plus in case the stock has been moved or consumed, it may cause some problems. Perhaps you can try to create an LCV for them and test it out.