https://forum.r31skylineclub.com/index.php
All your questions have been answered there
I still have my R31 which I bought 15 years ago! Spend the minimum amount on maintenance etc. Exhaust upgrades is meh, 2.25 with two good mufflers is plenty. Not worth on a standard RB30E.
Spend the money on bushes, better brakes, suspension, tyres, etc. Loads to do before touching the engine and exhaust.