I believe the part you're referring to would be the bumper reinforcement beam.
Item #18 on this illustration -->
http://www.gmpartswiki.com/getpage?pageid=88066
The original GM part number was 337470 but they have been discontinued for many years and are not being reproduced. It's also a 74 Nova specific part so finding a good used one may take a bit of searching.
Is your old one bent or twisted to the point where it's not lining up with the new bumper? If so, a shop with frame straightening equipment might be able to bend it back into shape.
Also, if your new bumper is a 73 style (with the wider air intake slots), it should still bolt up to the 74 inner reinforcement beam. But a couple of the bolt-in gussets (item #19 on the above linked illustration) will have to be removed because they will interfere with the slightly rolled in edges of those slots in the bumper.