What size radiator?

I've got a 1973 Duster that's powered by a 360 from a 75 duster in it. I need to put in a new radiator but want to get the proper size. The one in there looks like the core is about 26" wide and with the end tanks it's 22" tall. But in the engine bay, the last inch on each side is blocked by the metal panel holding up the rad, so I don't think it's doing much. I've seen 22" listed as the size for a 73 Duster as well, so which size do I need? I was looking at Champion rads so if anyone has a part number there that would work - or any other suggestions for a good manufacturer.

Also, anyone have experience installing an aftermarket temp gauge? Is it a hard swap, where'd you put the gauge, and do I need a new sender too or just the gauge? Would be nice to see a real temp reading.