If you have your parts in hand, the best way to check would be to install the new cassette on the freewheel (assuming you have a cassette lock remover) and check the clearance between the rear triangle’s dropouts. That way you know you have enough clearance for both chain and cassette. Next step would to be install the new rear derailleur (don’t install cable yet) and using your thumb, push the derailleur to it’s lowest point (the largest ring on the cassette) and make sure the jockey wheels and derailleur cage don’t touch the ring. That will ensure smooth operation without damage to the components. Hope that helps a little.
Peace from up here in FL 🙂