Hello Fiat,
Nice to have another Italophile on board,
Sorry no diagram but I have two custom built ones on two of my old cars. And there is no issue. The secret is that the guys who built it had the old one as a sample. They told me that this is important so that they know stuff like required capacity and area etc.
What is your car? Is there another that has the original which you might be able to show the guy who built your custome made radiator?