I'd suggest finding a friend familiar with wiring -- it is too difficult to explain on a forum, and the Starter circuit requires some heavy-duty wiring, as the Starter Motor draws ten's of Amps when turning over the engine. The Starter also utilizes a relay to control the heavy current draw, indirectly; if wired wrong, it could lead to a fire, very quickly...
The only place available to move the Battery would be the tail area, requiring longer cables. Some folks here have used a smaller Battery, in the original location; since you are planning on removing the signal lights, this is a possibility as there will be less draw upon the Battery, and the Alternator.
With regards to the back end, a lot of modification will be required before you can put a fat tire on it, as the bike was designed for a relatively narrow tire, and there just is not room for a really wide tire. It is not impossible, but I suspect it will require cutting and welding of the swing arm, removal of the rear fender, and cutting and welding of the frame to make room by moving the shocks out farther. Good luck! Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII