They'r 5 114, front tyres are 8J - 235 40 18 and rears are 9.5J - 265 35 18

Offset is ET20.. will these fit onto my z?

Should do, they may stick out a bit though.

They should fit perfectly without any probs unless of course you have a big brake kit fitted, then you may need spacers on the front.

Won't need spacers at the front and you won't really have any scope for fitting spacers with that spec. Fronts should be right out in the arches and will look pretty good. The rears however, 9.5" with ET20 won't fit at all. You'll need some big arch rolling modifications for that. With that width, you really have a max of +35. I'd double check the check offset of the rear as +20 on a 9.5 is pretty extreme.