[Mr70]

I deleted my sentence in earlier post,a 4 core wasn't possible with a 3:31/LS6.Sorry for the mis-information.
The Harrison Tanks are correct,and the Monte Carlo tanks will work,as long as they are an Auto car as yours is.
You might want to get the Rad re-cored though.
Suffix on 3 core rad tanks should be:
I A-on Drivers tank.
X I-on Passengers tank.

I have a 3 core Rad here now.You will also need the Correct 3 core radiator shroud for it.
This shroud is unique in that it is different from the 4 core shroud,and is not correctly repoped today.
I have one of those too.