| Shouldn't be a problem but remember that lower profile tyres give less flex and offer a harsher ride.
It may be better to keep the smaller wheels with bigger profile and more flex to give the box a smoother rider over the lumps and bumps.
But, you're not worried about changing the rolling radius (size of wheel + tyre) so as long as you can get a suitable wheel + tyre combination it should be ok.
Try ebay as a cheap source of steel wheels. | 
antonye
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