Assuming you don't have an original Harrison unit to work with, there are several choices;
US Radiator offers a quality product in several styles that work well for the BB Nova. You can choose either a 3 or 4 core unit or an aluminum 2 core...I have used them and they work well.
www.usradiator.com
http://www.usradiator.com/radiators/...ml?cat=458&p=2
I believe Camaro Specialties sells a repro Harrison unit...looks original...big bucks.
There are a variety of other aftermarket units out there...take your pick/perhaps others can comment on quality.
For the money, the US Radiator product is the best value.
-wilma