The Leslie 25 is directly compatible with the 26-1 box and half-moon switches. I am assuming you have the switches too! If you already have this kit hooked up to your organ, you are all set.
If you don't have the kit hooked up to your organ, see captain-foldback.com and look at the schematics for the 25 - it will illustrate how the 26-1 hookup kit should be installed into the organ. I hope you already have the large 5-pin harness tyhat allows you to intercept the speaker leads and take power from the organ. If not you will have to roll your own. Basically you just need a way to intercept the speaker lead and get power into the box.
Since you are hooking the 25 up to an organ, make sure you have the console load resistor in place on the 25 junction box.