If you can get any downtime at all to do this project, I would ask for it. It is better to ensure connectivity and stability than to cross your fingers and hope. But if you can't...
I would build one of the new Dell servers with a base OS install ready for cluster membership. Evict your secondary node in the production cluster and join the new Dell in it's place. Once that has been tested and running well, move the cluster services to the new Dell, test for a while to make sure nothing goes wrong, and then complete the process on the second node.
The only problem I see with this is that you will not be able to verify disk connectivity to the shared storage on the new servers until they are cluster resource owners. Also, you will need to ensure that any applications, such as SQL server, are correctly installed and configured to the new server before moving resources to it.