Im struggling to replace the radiator on Pontiac 1976 Grand Prix and advice would be great

What is the problem? Radiators are pretty simple.

Radiator is held in place simply by being mounted within the fan shroud......there are four black rubber mounts that the radiator sits in. Can not seem to get the rubber mounts to stay in place whilst juggling the radiator into the right position

Set the mounts into their receivers rather than leaving them on the radiator.

You could also insert the rubber mounts into place from underneath the car which should be a simple task.

they slip out and don't stay in place; battling with keeping the four of them in place at the same time whilst trying to spread the plastic shroud apart etc

I suggest a helper.

...tried the two from underneath, but once again when working on the top two the bottom two get displaced by the juggling around of the radiator.....is glueing them in an option?

ok, missus on her way

Don't the top two sit in brackets that bolt on? Did you remove those mounts?

The mounts don't bolt on, they just sit in the plastic work of the whole shroud assembly. The whole shroud bolts to the car with the radiator sitting within it; with the four mounts between the radiator and the shroud