I've also purchased Osco manifolds and other exhaust parts over the years and have never had a problem. While Osco sells through distributors, they will discuss applications over the phone and have been very helpful to me in finding the right part. For instance, I found out from them that I could replace the elbows on my V-Drive Chrysler 360s with their 20-degree down angle elbows that were shown in their catalog for a small block GM manifold. They fit perfectly.
I keep an Osco catalog as a great reference for exhaust parts.
Bill