How would 18x8.5 +35 go on a GC8?
Am I wasting my time or is it possible to squeeze them on.
I'm guessing lip the guards, some negative camber and maybe some stretched tyres?

There is possibly some way you may squeeze them in with neg camber etc...but the car will handle like shit...so the question is do you want to improve the car and make it handle well and look good....or do you just want to do it for looks alone?
Personally I would be looking for 18 x 8 with an offset of +48 to +55 ... and you can have the best of both worlds.

I have Prodrive WRC P1's 18x7.5 +46 offset with 225/40R18 Falken RT615 tyres on my 99 GC8.
DMS 40mm Coilovers with 3.5-4 degrees camber on the front. A bit less on the rear.

Rear Guards definately had to be rolled. Width of the tyre was the cause of this. A 215 width tyre and you would have no rear inner guard interferance problems.

A +35 offset even in 18 x 8 and I think you are definately going to have problems with the correct 225 width tyre.

As stated above +48 to +55 on a 225 18 x 8. +55 definately ok. +48 maybe.
The next problem is finding decent good value tyres in the correct load rating (89 or higher) for the 18's in a 225 width. To have the correct rolling diameter you need 225/35 R18 which are scarce as well.

Falken RT615's were the only high performance reasonably priced tyre in the correct load rating I could find without going to expensive Pirelli's. And even then the rolling diameter is not correct.

J

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"You can turn your back on a person, but, never turn your back on a drug. Especially when it's waving a razor-sharp hunting knife in your eye." - Raoul Duke

will 45 or 48 scrub on the inside?

18 x 8 +45 definately will scrub inside rear with 225 width tyres.
18 x 8 +48 would be right on the inside edge. maybe. close.

Again it depends on how lowered the rear is as well.

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"You can turn your back on a person, but, never turn your back on a drug. Especially when it's waving a razor-sharp hunting knife in your eye." - Raoul Duke

17x7.5" i mean sorry with 225/45R17

Thanks for the input, will have to look into some other rims then.