sub 5sec 0-100 and 300kmh Vmax, that is definitely achievable bro, especially with RB26... just make sure u get around 3.7-3.9 R200 longnose rear differential (some comes with OEM viscous lsd - enough for light track work n can support LOTS of power n torque). but it aint as easy as transplanting a bone stock rb26. definitely need to be re-worked. there are some rb26 sifoos in here willing to give some tips on building powerful+reliable monster rb26...
i pledge RM50 to this project, and if there's anything that i can be of assistance, please do let me know...
good luck!
p/s: here's the big boss of RB26 Z31... inspiration of all Z31 owners worldwide. owner's nickname is Little K.