I believe the stock 37 PP are +30 mm and the stock 35's are +34 mm. They are probably saying that because they require more space than the stock wheels as to not rub. I am all stock except a 1.5 perch collar so I don't have an answer. I gotta think someone one here has tried it. But I have heard that same message before.
How much space was recommended? 00 offset? That would take about a 1.3 inch spacer to get you to 00 mm offset. Or you can buy aftermarket wheels at 00 offset. I would stay away from going negative offset because that will cause other issues with the fenders.
I know back spacing and offset are different things. I do not know the stock backspacing. Most of the time offset is what matters. I would be interested in the answer as well.