you might get away without spacers on the rear, they're already gonna be half an inch further out, as well as half an inch further in. might help to keep the wheel centred in the well for maximum clearance. if you space it by 10mm then the outside edge will be 23mm further out than before, which is quite a lot really.
when i swapped 8.5" wheels with 9.5" wheels, they were noticeably wider, even with the same offset. i definitely think its worth bolting them on as is, seeing how they look (and then taking a photo :evil: )