The 3" cone won't just bole in place of the stock air box, the inlet is 2.5" or something like that. You need an adapter plate.
But if you get an AFM off an old Supra (82?), it has a 3" inlet ANd the flap is teh same size, so if you use a wider pipe to your intake you can expand your total inflow.
When I did the battery/AFm flip, I ran an 8-conductor wire back along thefirewall. Used some crimp-on connectors on both ends, so it could be connected/disconnected w/o any soldering or more cutting.