Pontiac Radiator Mounts

[Vimes]

You would need something like this to hold the radiator in place.

https://www.jegs.com/i/JEGS/555/52075/10002/-1

These are universal, not an exact fit, but will give you an idea of what you need. Your factory shroud has the same sort of retainer, only made of plastic and molded in. If you use these, you'll need to modify things to make them fit. If you have any metal skills at all you can probably make some.