I can tell you from experience that it's a bad idea to use 40 profile 18 inch tyres on an SW20.
On the rear it's bad due to excessive speedo inaccuracy.
On the front it's bad because you can't turn full lock without the tyres rubbing the wheel arches.

Work Emotion CR Kiwami
Front 17x7 et 38
Rear 17x8 et 35

Only test fitted fronts. Clears the Nissan Z32 bbk upgrade.

What if I stretch a 215 tyre on the 9.5 inch rim? Will it fit the front then? 

like Rob said stay away from a 40 series tyre for start.
 
I put in the numbers you quoted against specs for the front setup I have and this is what it came up with:
It would poke a further 18mm out of the guard and sit 10mm closer to the suspension.
 
Are you running coilover's? I have coilover's on mine and with my setup I only have 8mm clearance between my coilover and my rim and the rim is poking just out of the guard with running 2deg negative camber.

Yeah I'm abandoning the idea!

^If you were talking about that for the rear, then it might work. But 9.5 on the front of an MR2 is just not gonna happen without widebody fenders. Unfortunately, we can't match Nissans for fitment.

8.5" fronts is as large as I would go, and that already you need to be perfect on the offset and tyre size for it to work.

Hi.
Please comment on this proposed fitment, thanks:
F: 18 x 8.5 +30 with 215/35
R: 18 x 10 +35 with 255/35
King springs.
All guards rolled.
The front is 4mm from possibly touching the suspension.
 

Fronts will most likely scrub the inner wall when you're at full lock. at +23 for my fronts and then full lock it doesn't scrub.
Rears should be ok unless you hit a really bit bump and bottom out.

dylmrt
forgot to update this:

F: 18x8.5+27 2.5deg camber
R: 18x10+37

How was this set up on full lock?
Thanks for the reply.