your kit with longstroke work best with 1,5mm head gasket that you have to use. Be sure this head gasket is of the right diameter that should be 1mm larger then your piston, so has to be of 70mm.
The other thing you have to do is to change your timing no more then 21. Try 21 or 20.
If you stay at 21 use this in combination with a 9 grade NGK or 3 grade for Bosch or 27 if using Denso.
If you prefer it with time advance set around 20 you should use NGK 8, Bosch 4, Denso 24.
I personally prefer whenever possible to use less advance and a slightly hotter spark.
The engine will rev more free at high rpm.
As far as concern the fourth, it depends on which exhaust you are using. If you are using a PM, that is my favourite with this set up, you can use the PX200 fourth no problem. You can use it also with the standard exhaust or Sito plus. If you are using Leo vinci or Polini you should stay with the short fourth. Simonini OK with PX200 fourth gear too.
I hope your exhaust is one of these if not write again.