Lee(Leroy17), you should write a bible on making the cars more comfortable.
Looking at seat thickness is a good suggestion as it can give you gains everywhere - seating height, clearance to dash and steering wheel, distance to steering wheel, distance to gearshifter.
It is a little more aggressive, but you can gain space by also modifying the rear wheel arch in the rear bulkhead wall and also wall around where Skyline cradle connects to chassis. You can get the seat back so that the base of seat will virtually go all the way back to the cradle frame. Big gains there as well.