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I think you'll have some difficulty getting your phones working there because the radio frequencies and waveforms supported by your handsets aren't well supported by Japanese mobile towers. For instance, I looked up your Lumia 635 specifications and compared them to the frequencies used in Japan. The only thing that seemed to line up was 900 MHz HSDPA on SoftBank's network. So you may see some text messaging and limited data capability while you're near a compatible SoftBank tower, but maybe they aren't available or compatible where you happen to be.
Another option you might consider is renting a smart phone and SIM card from one of the providers listed at the above link.
I visited Japan for a few weeks last year and was able to use my Lumia 925 with T-Mobile services while there, but it was intermittent at times and largely unavailable while traveling (e.g. while on shinkansen). Newer phones, especially mid-to-high end ones (e.g. Lumia 950, iPhone 6, Galaxy S6), have significantly better radio support and will work much more frequently on overseas networks.
I agree that between the devices supported frequencies and the frequencies that Japan offers, it may be a challenge to get connected. You could try manually searching for networks and see if you're able to connect to Softbank. Calls and messages are most likely going to be how these phones will be able to use the service while you're there.