There is no such thing as “over cooling” an engine. That’s what the thermostat is for. It controls minimum operating temperature.
You are much better off with the best radiator you can fit in there and let the thermostat do what it’s designed to do rather than skimp on a radiator and fight cooling issues.
With that said, 1.25 tubes all day long and twice on Sunday. Do not skimp there. Find a way to make the rest of it fit.
