I have a similar set-up as you, except I don't have SI transducer on trolling motor. Most of the time, I just run with the 400TX in the bow instead of swapping units around. I am always afraid of dropping the 798 head. The 400tx head can be easily modified to plug into the newer connector style block. If you have a removable trolling motor, keep your 400TX mount in the bow. It has the machine screws and will hold up better from taking the transducer plug in and out.
The 400TX power cable will be fine. If you get noise, get a new cable.