I picked up a set of 15x7 5-spoke Z28 wheels and the guy said the rears have a different backspacing then the front, is this true? also if so which is which? it looks like on the part number on the back two end in 85 and two in 86

 

On the 15" 5 spoke aluminum wheels the back spacing is 4.25" and the offset is 0.The difference in the part numbers could be that the wheels maybe from different years.Those wheels were used from '82 to '92 on Camaros.They were the standard Z28 wheel '82-'87,standard IROC '88-89,and also came on the RS.And early '90s 1st Gen S-10s used them too.The 16" wheels used with the IROC package,first available in '85,have a different offset front set from back set but the wheels are marked on the spokes as to where they go.