Been there done that. First thing you should do is cover/seal that pod filter, or install standard airbox. Sucking in hot air does nothing positive for your engine.
Sell your 3" exhaust and buy a GTS1/2 style exhaust manifold (6 to 2 to 1). . Buy two fat mufflers with 2.25" ends, then take it to your nearest decent exhaust shop and ask them to make you an exhaust with mandrel bends. Sound done.
Get a 'hotter' camshaft and Nistune. Install and tune. Upgrade suspension and brakes. Replace all flogged out bushes (e.g. rear trailing arms). Also invest in a working LSD. Learn how to drive, go to track days etc.
Once you're confident, sell the car or go all out on a turbo build.
There's a whole forum dedicated to your car
https://forum.r31skylineclub.com/index.php
http://wiki.r31skylineclub.com/