trying to bridge C6300 to WNDR3400v2

It’s a little hard to follow the references (though I can understand why you did that). If I understand correctly you would effectively be reconfiguring the WNDR3700 as an Access Point connected by Ethernet to the C6300, and the WNDR3400 would act as a repeated to the ‘3700 as it did before. Since the ‘3700 – ‘ 3400 repeating previously worked then I can’t see a reason why it would not work as you suggest. I would advise that you keep a copy of the current firmware for each somewhere safe, since a future update could break that setup by removing WDS functionality.

Please let me know how you get on, if you run into any issues with your suggested arrangement I have a feeling that they should be resolvable.