Few questions on radiator tube size

Original Mopar radiators are brass not copper. Brass is a copper/zinc alloy.
Brass conducts heat almost 50% poorer than aluminum.

Don't beat me up. Above is from pg 55 of current Mopar Action magazine, June 2021.
You'll do fine with either 1" or 1-1/4" tubes but as already mentioned, pay attention to core thickness.