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you can fit 18x7.5 +45mm offset on all four, but not sure
about +26 on the rear....perhaps if you have a monster-truck
type of setup. or aftermarket fenders.
Looks like this is the setup you're after but I'm doubting that the rears are the 8" wide wheels with the 26MM offset. Not sure who own this MCS but you may want to locate the owner to discuss possible wheels size and offset options.
Why not use the 7.5s on all four corners. I think you may have some understeer problems with wider wheels in the rear. The low offset on the rear will push the wheels away from the suspension and under the wheel arches on the body of the car.