You didn't miss anything....I did. I didn't realize you had the W72 engine. I couldn't tell from the picture that it was an HB. The W72 engines came with the HB which was larger than the standard 400 hub (which is what I thought you had). The standard 400 hub is smaller (it doesn't have the rubber insert and the metal part outside the rubber insert) and uses a timing tab. The W72 engine has the timing marks on the timing chain cover. So, you're perfect!!
The W72 engine was designed for higher RPM's than the standard 400. I don't think the standard 400 would fair very well at higher RPM's.
By the way, GM still has that HB in stock for something like $30. I bought one as a spare for my car. They're made in India though.
By the way, you have a great car.