Admittedly the true GT5 cars had a stepped axle with larger outer bearing, but lots of people are running wide wheels on GP4 and GT5 conversions. You should be fine with new aftermarket axles, what probably happened is someone replaced the wheel bearings with an already own axle. Also be sure to replace the chamfered outer thrust washer as that can cause premature failure on an otherwise new axle and bearings too.
Julian
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