I think we will also need to know the current wheel offsets, because as far as I know you are adding material from the centre out, if you were getting new inners / outers it would be a different story.
I.e. if the 14 x 7 is 0 offset, and each inch is 25.4mm to achieve 8.5" the wheels offset will be 0 - the width of the new outer ( 25.4mm x 1.5 = 38.1) so the 14 x 7 0 would become 14 x 8.5 -38
I hope that makes sense![]()