Since you have had no replies so far, I will give some advice based on first principles rather than experience. (I have never tried to weld such large pieces of Cu together.)

The thickness given should be weldeable without a pre-heat, as long as you use a relatively high heat input process such as GMAW.

I think that your big problem is going to be buckling distortion. You could try to jig the welds with a very rigid jig, and then peen the weld before removing from the jig.

Regards
Niekie