I have used 245/40R18 on the front and 275/35R18 on the rear with excellent results on track days. The selection of tyres in 18" is MUCH better that 19" and the tyre sizes you are suggestion for 18x8 EOM front rim and 18x9" OEM rear rim are truly excessive.
In 18" you have the choice of Advan Neova AD08 as a semi-R spec, or Advan A048, Bridgestone RE55 series, Dunlop DZ series, Toyo R-specs etc. In 19", the choice is more limited.
Going with slight lower sidewall height will drop the CoG of the car, as well as the roll-centres and give you higher cornering limits.
With the standard spring/sway-bar setup of the Z34, I have no difficulty in running a smaller front than rear. If I was to move to bump/rebound adjustable dampers, adjustable sway-bars etc, then it is worth moving to same sized tyes front and rear, but that also implies new rims of identical width.
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