haha typed a large reply and closed the page :/
breifly :
gearbox xmember is fine like that , especially a prefabbed kit like maddat.
on mine : was a custom xmember but .sandwhich plated through the floor with roughly a 200x200 3mm plate on top.
engineer was fine with it..

i strongly recomend an engine dampner tho.
although you dont have the maddat booster like me , still isnt a nice gap.
while drifting my motor has torqued enough to snap the bleeder nipples off..
engine dampner or solid engine mounts will fix the problem tho.
just a heads up :)

also. s15 radiator fits perfect. excel bottom hose. unknown top hose..
but^ i did modify my motors top water outlet so it faces to the side not forward (might just be an s13 thing)
need any other fiddly info you cant find just ask me on fb bud.