Has anyone identified what type of
plastic is used in the side tanks
of the radiator ?

Polyethyleneglycostyrene.

You got the answer, also they can be purchased from AC Delco, pretty easy to change.

thanks allaboutspeed, Danno...

I can't seem to find anything about Polyethyleneglycostyrene
via a google search....

Any thoughts on repairing a hairline crack in one of these
tanks...

or is it smarter to do the remove and replace thing

Have a shop R&R the tank; cost should only be around $30.
Or upgrade with a new one.

Pete